Surveying and Digitizing

Primary Data Sources Measurements

Field → surveying Lab (not covered here)

Remotely sensed data already secondary?

Creating geometries Definitely in the realm of secondary

data Digitizing Scanning

Surveying Measurements and measurement

techniques Distances Angles Position determination

Applications Traversing and mapping Construction and earthwork Boundary surveys

Definition of Surveying General

To inspect, view, scrutinize, or examine To determine condition, situation, or

value

Specifically Science and art of determining relative

positions of points above, on, or beneath earth surface

Uses of Surveying

Locate/map resources Engineering design

Layout construction or engineering projects Verify performance

Acquire reliable data Provide control

Usually for location

History of Surveying Early applications

Boundary location Construction Mapping

Early surveys limited by technology Crude and inconsistent methods Development of sighting devices,

standards, …

History of Surveying (2) Industrial revolution improved surveying

Advances in available materials Improvement in tools

Electronics revolution fundamental advances Electronic distance and angle measurement Satellite surveying Enhanced processing

Specific Types of Surveying

Property (cadastral) surveying Control surveying Mapping surveying (planimetric or

topographic) Photogrammetric surveying Construction (engineering) surveying Route surveying Hydrographic surveying

Surveying Measurements

Two quantities measured in surveying Lengths Angles

All measurements are imperfect Errors Mistakes

Measurement Errors Sources of errors

Natural Instrumental

Types of errors Systematic Random

Terms used in describing errors Precision Accuracy

Personal

Idea of Relative Position

Question: Have the points moved? Answer: Relative to what? References

Needed for expressing location of points, lines, other objects

Datums provide references in surveying Horizontally Vertically

Reference Ellipsoids Basic Concept

b = semi-minor axis

f = flattening

aba

ab1f

a = semi-major axis

e = eccentricity

222

f2fa

bae

Example Reference Ellipsoids

Ellipsoid Equatorial Axis

Polar Axis Association

Clarke, 1866

12,756,412.8 m

12,713,167.6 m

NAD27 datum

GRS80 12,756,274 m 12,713,504.6 m

NAD83 datum

WGS84 12,756,274 m 12,713,504.6 m

GPS

ITRS 12,756,272.98 m

12,713,503.5 m

ITRF

GRS = Geodetic Reference SystemWGS = World Geodetic SystemITRS = International Terrestrial Reference System

Ignoring Earth Curvature

8000.000m ( 5 miles)

8000( 5

+ 0.25”).006m miles

998.95 km

1000 km

Distance

Ignoring Earth Curvature (2)

1 mile (1609 m)

8 inches ( 20 cm)

Level surface

Horizontal plane

Level line

Ignoring Earth Curvature (3)

75 mi2(48,000 acres)

19,800 hectares

Sum of Interior Angles =

180° 00' 01"

Triangle geometry

Digitizing and Scanning Instruments Georeferencing The process and problems

associated with it Automation Formats

Why Do We Have To Digitize?

Existing data sets are general purpose, so if you want something specific you have to create it

In spite of 20+ years of GIS, most stuff is still in analog form

Chances are somebody else has digitized it before; but data sharing is not what it should be

Digitizer

Digitizing table10” x 10” to 80” x 60”$50 - $2,0001/100th inch accuracy

Stylus or puck with control buttons

The Digitizing Procedure

Affixing the map to the digitizer

Registering the map

Actual digitizing In point mode In stream mode

Georeferencing at least 3 control pointsaka reference points or tics

easily identifiable on the map exact coordinates need to be

known East of Greenwich

72°71° 73°

72°71° 73°

11°

12°

11°

12°

Sou

th

Tic Points

Origin: X = 4 in. Y = 5 in.

Digitizing Table Coordinates

Entered: Tic 1: 11° 15' N 30° 30' E Tic 2: 11° 15' N 73° 30' E

Digitizing Modes Point mode

most common selective choice of points digitized requires judgment for man-made features

Stream mode large number of (redundant) points requires concentration For natural (irregular) features

Problems With Digitizing

Paper instability Humidity-induced shrinking of 2%-3%

Cartographic distortion, aka displacement

Overshoots, gaps, and spikes

Curve sampling

Errors From Digitizing Fatigue Map complexity

½ hour to 3 days for a single map sheet

Sliver polygons

Wrongly placed labels5 86 7

Digitizing Costs

Rule of thumb: one boundary per minute ergo:appr. 62 lines= more than one hour

Automated Data Input (Scanning)

Work like a photocopier or fax machine Three types:

Flatbed scanners A4 or A3 600 to 2400 dpi optical resolution $50 to $2,000

Drum scanner practically unlimited paper size $10k TO $50k

Video line scanner produces

vector data

Requirements for Scanning

Data capture is fast but preparation is tedious

Computers cannot distinguish smudges Lines should be at least 0.1 of a mm wide Text and preferably color separation

AI techniques don’t work (yet?) Symbols such as are too variable for

automatic detection and interpretation

Semi-automatic Data Input

(Heads-up Digitizing) Reasonable compromise between

traditional digitizing and scanning

Much less tedious

Incorporating your intelligence

Criteria for Choosing Input Mode

Images without easily detectable line work should be left in raster format

Really dense line work should be left as background image – unless it is really needed for automatic

GIS analysis; in which case you would have to bite the bullet

Conversion from Other Databases

Autocad .dxf and dBASE .dbf are de facto standards for GIS data exchange

In the raster domain there is no equivalent; .tif comes closest to a “standard”

In any case: merging data that originate from different scales is problematic – in the best of all worlds; there is no automatic generalization routine