Quality of TOPSAR topographic data for volcanology studies ...pmm/Rem_Sen_Lidar.pdf · Coincident lidar and TOPSAR data, as well as an abundance of other remote sensing data (e.g.,

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Remote Sensing of Environm

Quality of TOPSAR topographic data for volcanology studies at

Kilauea Volcano, Hawaii: An assessment using airborne lidar data

Peter J. Mouginis-Mark*, Harold Garbeil

Hawaii Institute of Geophysics and Planetology, United States

Received 27 September 2004; received in revised form 7 January 2005; accepted 11 January 2005

Abstract

We use airborne lidar data for the summit area of Kilauea Caldera, Hawaii, to explore the utility of topographic data collected by the

TOPSAR airborne interferometric radar for volcanology studies. The lidar data are processed to a spatial resolution of 1 m/pixel, compared to

TOPSAR with a spatial resolution of 5 m. Over a variety of fresh volcanic surfaces (pahoehoe and aa lava flows, ash falls and fluvial fans),

TOPSAR data are shown to have a typical vertical offset compared to the lidar data of no more than ~2–3 m. Larger differences between the

two data sets and TOPSAR data drop-outs are found to be concentrated around steep scarps such as the walls of pit craters and ground cracks

associated with the Southwest Rift Zone. A comparison of these two data sets is used to explore the utility of TOPSAR to interpret the

topography of volcanic features close to the spatial resolution of TOPSAR, such as spatter ramparts, fractures, a perched lava flow, and

eroded ash deposits. Comparison of the TOPSAR elevation and the lidar first-return minus the return from the ground surface (the so-called

‘‘bald Earth’’ data) for vegetated areas reveals TOPSAR penetration into the tree canopy is typically at least 10% and no more than ~50%,

although a wide range of penetration values from 0% to 90% has been identified. Our results are significant because they show that TOPSAR

data for volcanoes can reliably be used to measure regional slopes and the thickness of lava flows, and have value for the validation of coarser

spatial resolution digital elevation data (such as SRTM) in areas where lidar data have not been collected.

D 2005 Elsevier Inc. All rights reserved.

Keywords: Kilauea Volcano; TOPSAR; Lidar; Topographic mapping; Interferometric radar

1. Introduction

Digital elevation data are being used by the volcanology

community to study such attributes as the shapes of

volcanoes (Mouginis-Mark et al., 1996; Rowland &

Garbeil, 2000), lava flow volume (Rowland et al., 1999;

Rowland & Garbeil, 2000), lava flow rheology (MacKay et

al., 1998) and lava flow hazards (Glaze & Baloga, 2003).

Typically, these elevation data have been obtained from

interferometric radars, either from the airborne TOPSAR

instrument (Madsen et al., 1995; Zebker et al., 1992) or

from the Shuttle Topographic Mapping Mission (SRTM)

0034-4257/$ - see front matter D 2005 Elsevier Inc. All rights reserved.

doi:10.1016/j.rse.2005.01.017

* Corresponding author.

E-mail address: [email protected] (P.J. Mouginis-Mark).

(Farr & Kobrick, 2000). Evans et al. (1992) first tested the

utility of TOPSAR data for volcanology investigations at

Hekla volcano in Iceland, demonstrating that the thickness

of individual lava flows could be measured. Subsequently,

Mouginis-Mark and Garbeil (1993) used a TOPSAR scene

of Mt. Vesuvius (Italy) to study the erosional gullies on the

flanks of the volcano. Similar features on other volcanoes

(e.g., Ruapheu, New Zealand, and Mayon in the Philip-

pines) have also been imaged by TOPSAR because these

volcanoes are often prone to the production of debris flows

(called ‘‘lahars’’) that constitute significant hazards to the

local communities. The use of TOPSAR to study both the

geometry (width and depth) of the gullies, and the changes

in gradient downslope, constitutes significant advantages

over other field-based or air photography methods of

collecting topographic information.

ent 96 (2005) 149 – 164

P.J. Mouginis-Mark, H. Garbeil / Remote Sensing of Environment 96 (2005) 149–164150

There have been few studies that attempt to analyze the

quality of the TOPSAR data over different volcanic features.

Several studies of TOPSAR accuracy have been performed

in non-volcanic terrain (e.g., Lin et al., 1994); notably,

Izenberg et al. (1996) used field transits and an electronic

distance meter (EDM) to validate TOPSAR data for a flood

plain along the Missouri River. This flood plain was,

however, very flat and had very different micro-topography

characteristics compared to those of Hawaiian pahoehoe and

aa lava flows.

The ability of TOPSAR to accurately measure lava flow

thicknesses and identify flow paths for lava flows, lahars or

water is of great importance for understanding volcanic

processes. Rowland et al. (1999) performed a comparison

between TOPSAR data for Kilauea caldera and field-

derived elevation measurements from a total field station.

They concluded that TOPSAR data are useful for the

morphological and volumetric characterization of individual

volcanic features such as faults, lava flows, and cinder

cones, as well as large-scale flow fields. Their fieldwork

indicated that, in areas of sparse vegetation, the TOPSAR

elevations are good to 1–2 m. Multiple digital elevation

models (DEMs) collected over the same area at different

Fig. 1. Map of the recent lava flows and structural features within Kilauea caldera,

Lidar coverage is indicated by the rectangular box. HVO is ‘‘Hawaii Volcano Ob

Zone’’ and NP is ‘‘National Park’’ Headquarters.

times (perhaps years apart) could therefore be used to

measure changes in the volume of a volcanic landscape

(such as a flow field) provided that the data sets are co-

registered to a common datum and there is an absence of

trees. But in instances where recent lava flows have entered

forested areas (such as the Pu’u O’o lava flows of Kilauea;

Rowland et al., 1999), the lack of knowledge of the degree

of penetration of the TOPSAR signal into the canopy limits

the ability of investigators to infer the total thickness and,

hence, erupted volume of lava.

In January 2004, the Airborne 1 commercial lidar system

collected elevation data over Kilauea caldera. These data

permit a quantitative examination of the performance of the

TOPSAR system. The Airborne 1 data set we use here

provides elevation measurements that we have binned into

1 m pixels and have a nominal vertical accuracy of 2 cm.

Here we make the assumption that these lidar data provide

the ‘‘true’’ topography of the volcano. This assumption

appears reasonable because the footprint of the lidar is 30

cm and, typically, because of the multiple flight lines that

were flown over Kilauea caldera. Thus, any aircraft motions

that were present in a single flight line would have been

removed or suppressed during the creation of the lidar

Hawaii. Inset shows location of the study area on the Big Island of Hawaii.

servatory,’’ KMC is ‘‘Kilauea Military Camp,’’ SWRZ is ‘‘South West Rift

P.J. Mouginis-Mark, H. Garbeil / Remote Sensing of Environment 96 (2005) 149–164 151

DEM. Typically, there are 3–5 lidar shots within each of

the 1-m pixels we have created by weighted interpolation.

In this analysis, we use these lidar data to investigate the

vertical offset of TOPSAR data compared to the lidar

topographic data over fresh lava flows and forested volcanic

features on Kilauea Volcano, Hawaii. To perform a direct

comparison of data sets, the TOPSAR data (originally with

a 5-m spatial resolution) were re-sampled to the same 1-m

grid using a nearest neighbor approach.

2. Study area

Coincident lidar and TOPSAR data, as well as an

abundance of other remote sensing data (e.g., Ikonos

images), exist for the southern half of the summit caldera

of Kilauea volcano, making it an ideal test area for the

assessment of remote sensing instruments. In addition, a

great diversity of volcanic landforms can be found within

this part of the caldera. As shown in Fig. 1, there are

numerous lava flows that were erupted between 1897 and

1982. Most of these flows are of the radar-smooth pahoehoe

surface texture (Campbell & Shepard, 1996; Gaddis et al.,

1990), although segments of the 1971 lava flow comprise

radar-rough aa textures. We also include Halemaumau

Crater, which is ~900 m in diameter and has been the

source for several eruptions over the last century. Structural

features are also common near the summit, with fractures a

few meters wide associated with the southwest rift zone.

Also in the summit area, one can find lava flows that

traveled through rain forest (e.g., the 1971 flows) and many

exposures of the Keanakakoi ash that was created in 1790

by the explosive eruption of Halemaumau (McPhie et al.,

1990).

The summit of Kilauea volcano has also been used as a

test area for orbital and airborne radar data (Campbell &

Shepard, 1996), so that the validation of TOPSAR data has

broader applications that include planetary geology and

volcanology (Campbell & Campbell, 1992; Garvin et al.,

Fig. 2. Oblique view of the study area, generated by merging the lidar data with an

is towards the left of this view, and the trees are visible as the pebbly texture at

1981; Shepard et al., 2001). Detailed studies of lava flow

emplacement that rely on local topographic data have also

been conducted within the study area (Heslop et al., 1989).

3. Data sets used

TOPSAR data were collected on October 12, 2000

during the PacRim 2 mission, and a commercial company,

Airborne 1, collected the lidar data over a period of several

days in January 2004. Analysis of the results from TOPSAR

(Zebker et al., 1992) indicate that statistical errors are in the

2–4 m range, while systematic effects due to aircraft motion

are in the 10–20 m range. The Airborne 1 lidar is a scanning

system that collects 25,000 points/s. Range error from the

laser system is expected to be on the order of 5–7 cm,

independent of altitude. Data were collected from an altitude

of 2000 m. A full description of the accuracy of the

Airborne 1 lidar can be found at http://www.airborne1.com/

technology/LiDARAccuracy.pdf. Supplemental data sets

used here include a panchromatic Ikonos image (1 m/pixel)

collected on November 11, 2000, and numerous air and

ground photographs collected between 1984 and 2003.

Used in conjunction, the lidar and Ikonos data (Fig. 2)

clearly show the elevation difference between vegetated and

un-vegetated parts of the volcano.

3.1. Qualitative comparison of data sets

The quality of the lidar topographic data is superb for

geologic mapping the caldera. For example, in Fig. 3, we

show a shaded relief image of Halemaumau Crater that

shows many of the cooling features associated with the lava

lake that was active in 1974. Also visible at the foot of the

crater walls are numerous individual blocks that have fallen

off of the walls and ‘‘benches’’ (which are former levels of

the lava lake) mid-way up the walls. The smooth surface of

the tourist parking lot is also recognizable on the southern

rim of the crater. Numerous drainage gullies on the southern

Ikonos panchromatic image. View is from the southeast. Halemaumau crater

the right of the image. See Fig. 1 for the area of coverage.

http://www.airborne1.com/technology/LiDARAccuracy.pdf

Fig. 3. Shaded relief version of the lidar data for Halemaumau Crater, with

artificial lighting from the lower right (southeast). Numerous structural

features associated with the 1974 lava lake can be seen on the crater floor,

and cooling features around the perimeter of the crater floor are also visible.

The smooth area on the southern rim is a parking lot; in the original data,

individual cars can be identified within this area. North is towards the top of

the image.


part of the caldera rim, and the several block faults on the

western side of the caldera, can also be seen.

Shaded relief versions of both the lidar and TOPSAR

data reveal the differences in the quality of the two data sets

(Fig. 4). We now illustrate the quality of the lidar data with

five comparisons of the lidar and TOPSAR data that have

relevance to volcanology investigations at Kilauea or other

basaltic shield volcanoes around the world. In each

example, we also show one or more representative ground

photo and compare topographic profiles obtained by the two

instruments.

3.1.1. Spatter ramparts that formed during the April 1982

eruption

Here we test the ability of TOPSAR to measure the

height and width of small cones that were produced by gas-

rich lava during a fire-fountain eruption. The April 1982 line

of cones was created during a 19 hour-long eruption within

the caldera just to the east of Halemaumau crater (GVN,

1982a). Lava was erupted from an ENE-trending fissure ~1

km long, and prominent lava flow lobes formed to the north,

east and south. The volume of new lava erupted was

~0.5�106 m3 and comprised primarily pahoehoe flows.

The resultant spatter cones over the vents, which are

5–10 m high (Fig. 5c), are composed of welded spatter.

Analysis of the shape of such cones and the height to

which the fire fountain reached during the eruption have

been shown by Parfitt et al. (1995) to be good indicators

of the total volatile content of the erupting magma and the

angle of ejection of material from the vent. Thus, the

measurement of the height and shape of the cones

provides a measure of the eruption conditions. Compar-

ison of the lidar and TOPSAR data (Fig. 5d) shows that

the primary difference between the two data sets is the

inability of TOPSAR to measure the steep sides and

absolute height of the cone. An area of generally higher

(by ~2 m) relief is associated with the cones, but this

underestimates the 7-m height and ~30-m width of the

cone. While not common on Kilauea volcano, there are

other basaltic volcanoes where cinder cones <100 m in

diameter have been measured by TOPSAR (e.g., in

Savai’i, Samoa; Kallianpur & Mouginis-Mark, 2002);

geometric measurements of cone shape on these volcanoes

is expected to give a height that is too small and a width

that is larger than actually exists. Classification of the

degradation state of a population of cinder cones (e.g.,

Wood, 1980) would therefore suggest erroneously high

degradation states for the smaller cones because TOPSAR

would indicate that they are lower and wider than is

actually the case.

3.1.2. The 1974 lava channel on the south caldera rim

In this instance, we explore the use of TOPSAR to

identify the difference in elevation along a lava channel,

which has been used by Heslop et al. (1989) to estimate the

speed and volume of the lava, as well as its rheology at the

time of flow. This lava channel formed in July 1974 and was

unusual because the vents were located on the caldera rim.

Inspection of the TOPSAR data (Fig. 6b) reveals several

areas where the radar was unable to measure the rapid

changes in relief that are associated with the caldera rim and

the northern wall of Keanakakoi pit crater. A number of

preexisting fluvial channels were also located close to the

vents, and the fluid lava flows were able to exploit these

channels and flowed rapidly downslope onto the caldera

floor. The result of this fast moving lava flow was the

creation of a lava channel where the two sides are at

different elevations as the channel made turns while going

downslope (Fig. 6d).

Comparison of the lidar and TOPSAR data (Fig. 6e)

shows several additional problems that would affect a

volcanology investigation. First, a line of narrow spatter

ramparts on the caldera floor is not visible in the TOPSAR

data. Our observations suggest that TOPSAR should not be

used to map structural features and linear vent systems a few

tens of meters wide and only 2–4 m high. Second, the slope

angle of the caldera wall is underestimated by TOPSAR.

This could lead to the misinterpretation that the caldera

walls had experienced collapse or other form of erosion.

Finally, the lava channel studied by Heslop et al. (1989) is

not clearly defined in the TOPSAR data but instead is a

shallower depression with almost three times the width.

TOPSAR would not be an appropriate data set to use in

order to conduct flow modeling of the type conducted by

Heslop et al. (1989).

Fig. 4. Comparison of shaded relief images for the study area derived from lidar (top) and TOPSAR (bottom). Artificial illumination is from the right in both

images. See Fig. 1 for distribution of surface units. White rectangles mark the locations of the sub-scenes shown in Figs. 5–9, and letters in top image locate the

sub-areas that were selected for TOPSAR penetration into the tree canopy (Fig. 12). TOPSAR data take is JPL No. 225-1, collected on October 12, 2000, at a

heading of 224.2-.


3.1.3. Drainage channels on the southern caldera rim

The use of TOPSAR to measure surface flow on shallow

slopes, and the degree of erosion of ash deposits can be

investigated on the southern rim of the caldera, where

numerous fluvial channels have formed in the Keanakakoi

ash in 1790 (McPhie et al., 1990). Our field observations

(Fig. 7d) indicate that these channels are typically 2–5 m

deep and up to ~20 m wide and possess sharp edged walls

where cemented ash has formed hard layers. This area

provides a good test for TOPSAR data where they are to be

used in surface flow models, such as the one developed by

Glaze and Baloga (2003), which can be used either for water

runoff or for the emplacement of new lava flows. Such

models have relevance not only for understanding the

emplacement of lava flows, but also for the assessment of

downslope hazards.

Comparison between the lidar and Ikonos data (Fig. 7a

and c) shows that the lidar data clearly illustrate the flow

direction for the water. However, the TOPSAR data (Fig.

7b) reveal only the most general drainage pattern, which

is further illustrated by a topographic profile across this

area (Fig. 7e). Despite a surface made of ash (which

might produce less of a radar return than solid lava), the

TOPSAR data reproduce the general characteristics of the

lidar profile, but miss the high-frequency (<5 TOPSAR

pixels) topographic features. Drainage channels are either

missed entirely, or their widths are overestimated. Such

limitations are expected to dramatically affect any

numerical model that predicts surface flow directions

and thus indicate that TOPSAR should not be used to

map drainage patterns at the scale found in this part of

Kilauea volcano.

a b

c

B

A

1130

1125

1120

1115

11100 100 200 300 400 500 600

Distance along profile (m)

Ele

vati

on (

m)

A B

d

Fig. 5. Views of the April 1982 spatter ramparts. See Fig. 4 for location. (a) Top left: Lidar shaded relief (in this and all other shaded relief images the

illumination from the right). The white rectangle marks the location of the ground photo shown in (c) and the line ‘‘A’’ to ‘‘B’’ denotes the profile displayed in

(d). (b) Top right: TOPSAR shaded relief. (c) Bottom: Ground photo of a segment of the spatter rampart [see (a) for location]. Person sitting on the rampart

crest provides scale. (d) Comparison of elevation data derived from lidar (dashed line) and TOPSAR (solid line). The location of the spatter rampart is marked

by the arrow. The position of the profile is shown in (a).


a b

d

e

c

B

A

LC

SR

BA

Ele

vati

on (

m)


1150

1140

1130

1120

1110

1100

10900 100 200 300 400 500

Fig. 6. Views of the southern edge of Kilauea caldera where the July 1974 eruption took place to form a lava channel on the caldera rim. See Fig. 4 for location.

(a) Top left: Lidar shaded relief image. The line ‘‘A’’ to ‘‘B’’ denotes the profile displayed in (e). (b) TOPSAR shaded relief image. Note the inability of the radar

to measure the height changes over the rim of the caldera and the wall of the pit crater at lower right. (c) Air photo looking north across the same part of the

caldera that is shown in (a). Black dot marks the location of the ground photo shown in (d). (d) Ground view of the central portion of the July 1974 lava

channel. Note the super-elevated rim of the channel on the far wall behind the person. Location of photo shown in (c). (e) Comparison of elevation data derived

from lidar (dashed line) and TOPSAR (solid line). ‘‘SR’’ identifies the spatter rampart that is missed by TOPSAR, and ‘‘LC’’ is the lava channel. Location of the

profile is shown in (a).


3.1.4. The September 1982 lava flow

Correlating elevations across kilometer-scale distances

has value when investigating the surface features that are

formed when lava is trapped, or ‘‘ponds,’’ behind topo-

graphic obstacles. The September 1982 lava flow is such

an example. Located in the southernmost part of the

caldera floor, this flow was created during a 15 hour-long

eruption (GVN, 1982b). The vent system was oriented in

the usual ENE direction, nearly parallel to the nearby

caldera wall. Field exposures indicate that there was

significant drain-back of lava into the vents after the gas

pressure was released during the eruption. An early

estimate suggests that perhaps as much as 3–4�106 m3

of pahoehoe lava was erupted. Of this, possibly as much as

1–2�106 m3 drained back into the vent system (GVN,

1982b). The resulting collapse of the pond surface left a

a

c

b

d

BA

1132

1130

1128

1126

11240 100 200 300 400 500 600


Ele

vati

on (

m)

BA

e

Fig. 7. Views of drainage channels carved in ash deposits to the south of the caldera rim. See Fig. 4 for location. (a) Lidar shaded relief image. Note that

channels can be identified at several scales in this image. The line ‘‘A’’ to ‘‘B’’ denotes the profile displayed in (e). (b) TOPSAR shaded relief image. Only the

caldera rim (at top left) can be identified in this image. (c) Ikonos panchromatic image of drainage channels. Note that the albedo of the channels (caused by the

amount of sediment on the channel floor) is quite variable, but does not correlate with depth of the channel as identified in (a). (d) Ground photo of a channel in

the general vicinity of the area shown in this figure (the exact location of this image was not recorded). Note that the channel is ~3 m deep, and that the

uppermost layers comprise layered, cemented, ash. (e) Comparison of elevation data derived from lidar (dashed line) and TOPSAR (solid line). Location of the

profile is shown in (a).



bathtub ‘‘ring’’ on the order of 2–3 m high on the

enclosing escarpments.

A key attribute of the September 1982 flow is that the

flow ponded within the topographic depression, creating a

very flat surface to the flow that is atypical for basaltic

volcanoes. This unusually flat surface has been used by

planetary volcanologists to predict the radar backscatter

characteristics of lava flows on Venus, as measured by the

Magellan and Arecibo radars (Campbell & Campbell, 1992;

Shepard et al., 2001). Despite the small hills that formed

over the vents as the lava level fell (Fig. 8d), which

A

B a

ce

d

c

c d e f

BA

0

1118Ele

vati

on (

m)


1122

1126

1130

100 200 300 400 500 600 700

Fig. 8. Views of the September 1982 lava flow on the southern edge of the caldera

‘‘B’’ denotes the profile displayed in (f). (b) TOPSAR shaded relief image. The c

photo looking north showing the September 1982 lava flow. The locations of the

photo of the largest mound preserved over one of the vents for the September 1982

of the caldera rim down which the September 1982 flow traveled. Note people in th

data derived from lidar (dashed line) and TOPSAR (solid line). Location of the prof

well as the top of the mound over the vent (‘‘d’’), are at the same elevation, conf

TOPSAR might have missed due to their narrow (<20 m)

width, there is a good match between the lidar and TOPSAR

data (Fig. 8e) for the main surface of the lava flow. Thus, we

are confident that TOPSAR on its own could be used to

determine the topographic roughness of lava flows such as

the September 1982 flow in Hawaii as an aid to the analysis

of planetary radar data. In addition, it is clear that TOPSAR

correctly shows that the two sides of the lava flow, the

elevation of the channelized lava flow that flowed through a

topographic low on the southern side of the flow (Fig. 8f),

and the top of the constructs over the vent, are all at the

b

d

e

rim. See Fig. 4 for location. (a) Lidar shaded relief image. The line ‘‘A’’ to

aldera rim can be identified from its shadow in this image. (c) Oblique air

field photographs shown in (d) and (e) are denoted by letters. (d) Ground

flow. See (c) for location. (e) Ground photo of the lava channel to the south

e middle distance for scale. See (c) for location. (f) Comparison of elevation

ile is shown in (a). Note that the two sides of the lava flow (‘‘c’’ and ‘‘e’’), as

irming that the flow surface was once at a higher level.


same elevation. This confirms that, when active, the lava

flow was originally at a higher elevation and that drain-back

into the vent area has indeed taken place.

3.1.5. The fractures associated with the SW Rift Zone

Structural features such as fracture and faults are

frequently used to investigate the recent deformation of a

volcano or the formation of specific features such as pit

craters (Okubo & Martel, 1998). One of the clearest

examples of fractures at the summit of Kilauea volcano is

the set of fractures that form the start of the SW Rift Zone. At

least seven parallel fractures up to 400 m in length, ~13 m

wide and 8–10 m deep can be found in this area (Fig. 9c and

d). Mapping the occurrence and length of these fractures has

been one of the goals of two Space Shuttle Radar experi-

ments (Gaddis et al., 1989; Mouginis-Mark, 1995) because

the results have immediate application to the interpretation of

structural features on Venus using the Magellan radar (e.g.,

Bleamaster & Hansen, 2004; Kaula et al., 1992).

Topographic profiles across these fractures (Fig. 9e)

reveal the inability of TOPSAR to resolve depressions of

this size. Either the fracture is not detected, or the width is

over-estimated by a factor of ~5 (i.e., the original resolution

difference of the two sensors). The absolute depth of the

fracture (>10 m) is also under-estimated by TOPSAR by a

factor of ~5. A comparison with the lidar data (Fig. 9a)

demonstrates the inability of TOPSAR to identify both the

length of these large fractures and the location of the

narrower examples. However, the shaded relief version of

the TOPSAR data (Fig. 9b) does at least allow the trend of

the larger fractures to be identified. Thus, we would not

recommend the use of TOPSAR for detailed structural

studies on volcanoes, although at a larger scale (i.e., features

at least several tens of meters wide or high, such as the

caldera wall; Fig. 9b), the data set would be adequate.

In summary, these five qualitative comparisons show that

care must be used when interpreting TOPSAR data of small

volcanic landforms. Two key attributes of the TOPSAR data

must be considered in any analysis: (1) the inability to

measure elevations in steep terrain, such as the walls of pit

craters or the rim of the caldera (Fig. 6); and (2) a smoothing

of the topography (at a horizontal scale of ~20 m) that

precludes the accurate measurement of spatter rampart

heights (Fig. 5d), the flow direction of fluvial channels

(Fig. 7b) or the depth of fractures along the SW Rift Zone

(Fig. 9e). With these caveats, TOPSAR data should be

adequate for most topographic studies of volcanoes,

particularly over a large area such as that employed for

‘‘whole volcano’’ studies of the regional distribution of

slopes on a shield volcano (Mouginis-Mark et al., 1996;

Rowland & Garbeil, 2000).

3.2. Quantitative comparison of data sets

One of the most important uses of TOPSAR digital

elevation data for volcanology investigations is the deter-

mination of the total volume of lava erupted. Often, this

determination relies upon the measurement of flow thick-

nesses where lava has invaded a forest, but without a good

knowledge of the degree of radar penetration of the tree

canopy only a poor estimate of lava flow thickness can be

obtained. Such a situation was encountered by Rowland et

al. (1999) in their investigation of the lava flows from the

Pu’u O’o cone of Kilauea, with the quantitative determi-

nation of a typical amount of TOPSAR penetration into the

forest of greatest importance where fresh aa lava flows

(typically ~10–15 m thick) have cut through a forest. To

address this issue, we attempt here a quantitative compar-

ison of the lidar and TOPSAR data for Kilauea caldera with

a specific focus on the determination of TOPSAR pene-

tration into typical Hawaiian forest.

Both the lidar and TOPSAR data sets used in this

investigation were collected with GPS ground control and

the data are referenced to the WGS84 datum. Thus, we have

found that there is no need to manipulate or adjust the

surface of one data set relative to the other. A comparison of

the difference between the data sets (Fig. 10) reveals that in

general the height offset is of the order of 2–5 m. Closer

inspection of the difference image reveals that there are

systematic height variations, seen as bands on the order of

~1 m across the floor of the caldera (Fig. 11a). We note that

these bands of elevation difference are neither in the

TOPSAR bearing’s along-track or cross-track direction.

Such errors have also been detected in other TOPSAR data

sets (e.g., the TOPSAR mosaic of Isabella Island, Galapa-

gos; Mouginis-Mark et al., 1996) and are most likely due to

uncompensated aircraft motions during data collection. As

inferred from field measurements across the caldera (Row-

land et al., 1999), most of the TOPSAR height differences

over bare lava flows are in the ~2 m range. A profile across

the 1919 and 1974 lava flows (Fig. 11a) reveals that almost

all of the height difference between the two data sets is <3

m. Comparison of this profile with the map of lava flows

(Fig. 1) shows that changes in the height difference (from

positive to negative) are not associated with real changes in

surface morphology.

In contrast to the bare lava surfaces, a profile across the

forest in the eastern portion of the study area (Fig. 11b)

reveals height differences of T5–8 m. Indeed, elevation

differences as great as ~12–15 m can be seen over forested

areas in the eastern part of the study area (Fig. 10b). Such

differences are most likely due as much to the difference in

spatial resolution of the two data sets as to the penetration of

the TOPSAR radar signal into the canopy of the trees.

Because of the considerable diversity in the height and

morphology of the trees in the Hawaii National Park

(Mueller-Dombois et al., 1981), a 30 cm lidar footprint is

quite likely to reach the ground surface without encounter-

ing a tree branch, while the 5 m TOPSAR footprint will very

frequently encounter both the top of the trees and the under-

story foliage above the ground surface. Such blocking of the

TOPSAR signal will also occur more frequently than with

a b

dc

A

B

e

BA

Ele

vati

on (

m)


1162

1160

1158115611541152115011481146

0 100 200 300 400 500 600

Fig. 9. Views of the fracture system at the start of the SW Rift Zone. See Fig. 4 for location. (a) Lidar shaded relief image. White rectangle marks the location of

the air photograph in (c). The line ‘‘A’’ to ‘‘B’’ denotes the profile displayed in (e). (b) TOPSAR shaded relief image. Only the edge of the caldera (at right) and

some of the wider fractures can be identified in this image. (c) Oblique air photo of the segment of fractures identified in (a). The two-lane road provides scale.

Black dot denotes location of ground photo in (d). (d) Ground photo of one of the fractures along the SW Rift Zone, looking southwest. People at bottom of

fracture provide scale. (e) Comparison of elevation data derived from lidar (dashed line) and TOPSAR (solid line). Location of the profile is shown in (a).


the lidar measurements due to differences in the incidence

angle of the two sensors. The lidar data are collected with a

scan angle of nadir to 20- off nadir, whereas TOPSAR data

are collected at 25- in the near-range and 60- in the far-

range. Spatial variations in the difference between the lidar-

measured elevation and the TOPSAR data may be due not

only to the type(s) of vegetation in a specific area, but also

to the maturity (and, hence, spacing) of the trees.

To mitigate the effect of the spatial resolution of the two

data sets, we reprocessed the lidar DEM data to a spatial

resolution of 5 m (i.e., the same as the original TOPSAR

data). Only the first lidar return, which corresponds to the

highest part of the tree that is measured in each pixel, is

included in these reprocessed data. The maximum elevation

within each 5 m grid location is assigned to that grid location.

We compare this value to the TOPSAR elevation data (Fig.

Fig. 10. Quantitative comparison of TOPSAR and lidar data for the study area. (a) Absolute difference in elevation between the first pulse of the lidar and

TOPSAR. Scale bar at bottom left shows that differences range from �5 m to +15 m [note that the intervals here are different from those presented in (b) and

(c), although the color sequence is the same]. The profiles A–B and C–D shown in Fig. 11 are marked. (b) Elevation difference between TOPSAR and the

bald Earth lidar returns. The greatest differences occur over the forested areas in the eastern (right hand) part of the image. Maximum differences are on the

order of 15 m. (c) Difference between the first return for the lidar data, smoothed to 5-m spatial resolution, and the TOPSAR data. This provides an elevation

difference that varies from near zero over the bare lava flow surfaces to >12 m in the forested areas.


Hei

ght

Dif

fere

nce

(m)

Hei

ght

Dif

fere

nce

(m)

0-1-2-3-4

500 1000 1500 2000

5

0

-5

500 1000 1500 2000

Distance Along Profile (m)

Distance Along Profile (m)

B

DC

A

Fig. 11. Direct comparison at the pixel-by-pixel scale between the elevation data presented in Fig. 10a for bare lava surfaces (a) and forest cover (b). Profile A

to B shows that across bare lava flows the difference between the two data sets (lidar first pulse minus TOPSAR) is never more than 4 m, and typically is <3 m.

Profile C to D across part of the rain forest shows that height differences of the order of 5–8 m are common across trees. The two segments of the profile where

differences are <2 m correspond to places where the July 1974 lava flow cuts through the trees in two separate lobes.


10c). In this instance, all of the TOPSAR pixels have a lower

elevation than the lidar image. Because of the comparable

spatial resolution of the two data sets, and the fact that we

selected maximum values from 5�5 pixel boxes for the lidar

value, the differences between the elevations is now smaller

and typically ranges from 8 to 10 m, with maximum values

<12 m.

Considerable theoretical work has been conducted to

assess the ability of TOPSAR to measure tree heights

(Kobayashi et al., 2000). By inspecting the lidar data across

the forested area of Kilauea Caldera, it is also possible to

determine the extent to which the TOPSAR measurements

penetrated into the forest canopy. We use the following ratio

to define radar penetration:

Penetration ¼ 5 m first return� TOPSARð Þ½= 5 m first return� bald Earthð Þ�4100%

Where the ‘‘5-m first return minus TOPSAR’’ yields the

penetration relative to the top of the tree, and the ‘‘5-m first

return-bald Earth’’ provides the approximate tree height.

In Fig. 12a, we show the general trend of the degree of

penetration into the entire forest. We use only a representative

sample of the entire data set in this figure, selecting every

625th data point in the full data set (i.e., one pixel within a box

measuring 25�25 pixels) to avoid saturating the graphical

display. A wide range in tree height (1–23 m) can be

identified from this sample, and the degree of radar

penetration into the canopy extends over the entire range

from 0% to 100% (Fig. 12a). There is a clear clustering of data

from trees that are 6–18 m high and a degree of penetration

that ranges from 10% to 80%. Striking differences in the

spatial distribution of these attributes suggest variations in

canopy morphology. We illustrate this point using three small

test areas (Fig. 12b–d), each 10,000 m2. We note that the

trend in Fig. 12a is different from the other graphs because the

smaller trees allow for a higher percentage penetration than

the larger trees. It is also possible that there are different

vegetation types within the entire forest that do not show up in

the smaller sub-regions selected for Fig. 12b–d.

Area 1 (Fig. 12b) has the shortest trees (2–12 m high)

and displays the greatest range of penetration values, with

many samples within the 30–75% range. Area 2 (Fig. 12c)

has the ‘‘cleanest’’ distribution of % penetration vs. tree

height and covers the smallest range of height (10–16 m)

and penetration (10–50%). There is a systematic increase in

penetration with increasing height. This trend is generally

similar to that of Area 3, but in Area 3 (Fig. 12d), the

100

80

60

40

20

0

100

80

60

40

20

0

100

80

60

40

20

0

100

80

60

40

20

0

0 5 10 15 20 25

0 5 10 15 20 25 0 5 10 15 20 25

0 5 10 15 20 25

a) b)

d)c)

First-Bald (m)

First-Bald (m) First-Bald (m)

First-Bald (m)

(1st

TO

PSA

R/1

st B

ald)*10

0

(1st

TO

PSA

R/1

st B

ald)*10

0(1

st T

OP

SAR

/1st

Bal

d)*10

0

(1st

TO

PSA

R/1

st B

ald)*10

0

Fig. 12. Plots of tree height (lidar first return minus the bald Earth measurement) against estimated TOPSAR penetration into tree canopy. Percent penetration is

defined as the [(5-m first return-TOPSAR)/(5-m first return-bald Earth)]*100%. See text for further discussion. (a) Plot of tree height vs. percent penetration for

a subset of the entire area, selected by taking every 625th data value in the area. (b–d) Examples of tree height vs. percent TOPSAR penetration for sample

areas within the forest. Each area is 10,000 m2. See Fig. 4 for the location of each area.


maximum penetration is ~40% and there is a greater number

of trees in the 12–18-m height range where penetration is

~10–20%. The highest trees (~20 m) are found in Area 3,

and TOPSAR has a penetration of ~25–40% in these trees.

These differences in TOPSAR penetration are surprising

and suggest that important ecological information could be

derived from the combined lidar and TOPSAR data sets;

however, such studies are outside the scope of this

volcanology study, and we leave such an analysis to a

future investigation.

Penetration values such as the ones illustrated in Fig. 12a

may not be true for all types of forest where TOPSARmay be

used to study lava flows, but they may provide an

approximation for refining the estimate of lava flow thick-

nesses on other parts of Kilauea volcano where new flows

have entered forest, such as the Pu’u O’o flows. In this area,

individual lava flows may be of the order of 6–13 m thick

(Fink & Zimbelman, 1986) and are thus considerably thicker

than the height error of the TOPSAR system. Knowing that

TOPSAR elevations for the forest adjacent to the new lava

flows have a known penetration percentage of the height of

the trees, it would be a relatively simple task to obtain a few

field measurements of tree height at the perimeter of the flow

and thus calculate the actual ground surface measured by

TOPSAR. Such a recalculation of the volume of the flows

investigated by Rowland et al. (1999) is beyond the scope of

this investigation, but this calculation illustrates the value of

better understanding the capabilities of this interferometric

radar system.

4. Conclusions

Our analysis has confirmed the value and limitations of

TOPSAR data for several types of investigation of basaltic

volcanoes such as Kilauea. In particular, TOPSAR data

appear adequate for comparing point-to-point elevations of

surfaces such as the perched September 1982 lava flow

(Fig. 8). The results of this study provide high confidence

in the accuracy of the TOPSAR system at the 1–2 m

vertical scale, thereby increasing the utility of the TOP-

SAR system for the validation of other DEMs. DEMs

produced from orbital radar interferometry, either using

repeat-pass data sets such as those collected by the ERS-1

spacecraft (Burgmann et al., 2000; Zebker et al., 1994) or

single-pass interferometers such as the Shuttle Radar

Topographic Mapping (SRTM) mission (Farr & Kobrick,

2000), need to be validated. Because of the relatively large

image size of a TOPSAR data take (typically 10�60 km),

it is possible to co-locate TOPSAR data with orbital data

that may have a spatial resolution between 30 and 90 m/

pixel. The TOPSAR data used here have been processed at

a spatial resolution of 5 m/pixel, so that the spatial

resolution of the two topographic data sets may differ by a

factor of 6–15.

This comparison of two DEMs at different scales will

not only permit an evaluation of sensor performance, but

will also facilitate new quantitative modeling of surface

processes that cover areas that are larger than a typical

lidar acquisition (say, >50 km2). Glaze and Baloga (2003)


have developed numerical models that predict the flow

paths of lava flows across the flanks of a volcano. Using

a combination of lidar and TOPSAR data presented here

for Kilauea as a test case for the sensitivity of the model

to meter-scale undulations in surface would give greater

confidence to model predictions based exclusively either

on TOPSAR or SRTM data.

We also note the potential value of the combined lidar

and TOPSAR study that is reported here to the ecology

community. There are clear spatial variations in the amount

of canopy penetration that has been achieved with TOP-

SAR (Fig. 11). We infer that these differences are related

either to the morphology of the canopy, the maturity or

type of trees, or the spacing between individual trees.

Using quad-pol radar data, attempts have been made to

characterize forests to better understand their biomass (e.g.,

Dobson et al., 1995; Harrell et al., 1997). Similarly, lidar

systems are also being used to characterize tree morphol-

ogy (Blair et al., 1999; Dubayah & Drake, 2000). Our

results show that additional characterization of forest

canopies could be achieved using the combination of

TOPSAR and lidar.

Finally, there is the value of a high-quality DEM as a

‘‘benchmark’’ in a volcanic area where topographic change

is very common, either due to a new eruption or to the

collapse of a crater such as Halemaumau. As demon-

strated by Rowland et al. (1999, 2003), the subtraction of

one DEM from another to obtain volume information of

an individual lava flow opens the possibility of under-

standing the rate of magma production during a specific

event. Comparing the thickness of a new lava flow as a

function of slope enables the flow properties (i.e.,

rheology) to be studied. This ‘‘before and after’’ topo-

graphic comparison is not only important for lava flows

because, given the large (>1 km) amount of vertical

movement of the floor of Halemaumau Crater over the

last 160 years (Walker, 1988), it seems likely that

topographic change will occur here over the next decade

or so. This topographic change could either be associated

with slow subsidence of the floor due to withdrawal of

magma down rift towards the on-going eruption site at the

Pu’u O’o cone (Heliker & Mattox, 2003, and references

therein), or to the occurrence of a new summit eruption

such as the April 1982 event. For the summit of Kilauea

Caldera, the Airborne 1 data are increasingly likely to

serve as a critical ‘‘before event’’ data set as future activity

occurs.

Acknowledgements

We thank two anonymous reviewers for their comments

on an earlier version of this manuscript. This research was

supported by a grant NAG5-13729 from NASA’s Natural

Hazards Program. This is HIGP Publication 1386 and

SOEST Contribution 6595.

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Documents

Quality of TOPSAR topographic data for volcanology studies ...pmm/Rem_Sen_Lidar.pdf · Coincident lidar and TOPSAR data, as well as an abundance of other remote sensing data (e.g.,