Photogrammetry

CE 426

Introduction

►Definition of Photogrammetry: the art, science, and technology of obtaining

information about physical objects and the environment by photographic and electromagnetic images.

Basic Information

►Mapping from aerial photos is the best mapping procedure yet developed for most

large projects.

� Used successfully for maps varying in scale from 1:1,000,000 1:120 with contour intervals as

small as 1 foot.

� Topographic mapping is the most common

form. – U.S.G.S updated and done this way.

� Used to reconstruct a scaled 3-dimensional

optical model of the lands surface using a stereoplotter.

Basic Information

►Uses: Aerial photos � Aid: geological investigations, soil surveys, land surveys, tax mapping, reconnaissance and military intelligence, urban and regional development, transportation system investigations, quantity estimates, shore erosion, etc.

� Mathematical methods have been developed to make precise 3-dimensional measurements from photos. ►Phototriangulation: 3-dimensional positioning of survey stations.

Basic Information Continued

� Photo has been used to take geometric measurements of human bodies, artificial

human hearts, large radio telescopes, ships,

dams, buildings and very accurate reproductions.

►In general it is not economical for small projects – the cost break even point is somewhere between 30 – 100 acres

depending on the situation.

Basic Information

►Photogrammetry can not be used successfully over the following types of

terrain.

� Desert or plains areas, sandy beaches, and snow – the photograph as uniform shades with

little texture.

� Deep canyons or high buildings that conceal

ground surface.

� Areas covered by dense forest.

2 Basic Categories

►Metrical photogrammetry – obtaining measurements from photos from which

ground positions, elevations, distances, areas, and volumes can be computed and topographic or planimetric maps can be

made.

►Photo interpretation – evaluation of existing features in a qualitative manner.

Types of Photogrammetry

►Aerial – series of photographs of an area of terrain in sequence using a precision

camera.

►Terrestrial – photos taken from a fixed and usually known position on or near the

ground with the camera axis horizontal or nearly so.

►Close range – camera close to object being observed. Most often used when direct

measurement is impractical.

History

► The first use of photogrammetry was by Arago, a French geodesist, in 1840. This included

topographic and terrestrial.

► The first aerial photogrammetry was by the French

in 1849 using kites and balloons.

► Laussedat (French) – father of photogrammetry.

► 1st in N. America – Deville, Surveyor General of Canada.

►U.S.G.S. adopted photogrammetry as mapping process in 1894 – mapping border between

Canada and Alaska.

History

►Airplanes brought great change to photogrammetry. � 1st used in 1913.

� Used extensively in WWI – photo interpretation.

� Used in WWII – mapping for recon and intelligence.

►WWII – 1960 – used often, expensive and accuracy problems for engineering design.

►After mid 60’s – advent of computer and plotting has made photogrammetric mapping accurate and affordable.

Photogrammetry for Engineering

►Defined: Photogrammetry is the process of measuring images on a photograph.

►Modern photogrammetry also uses radar

imaging, radiant electromagnetic energy detection and x-ray imaging – called remote sensing.

Basic Categories of Photogrammetric Interpretation

►Metrical Photogrammetry – obtaining measurements from photos from which ground positions, elevations, distances, areas and volumes can be computed and topographic or planimetric maps can be made.

►Photo interpretation – evaluation of existing features in a qualitative manner – timber stands, water pollution, soils, geological formations, crops, and military interpretation.

Geometry of Photographs

►Orthographic projection – each point projected normal to reference plane.

►Perspective projection – each point

projected through a central point, due to points being at different elevations, they

look 3 dimensional.

►Principal point (center of photo) – located at the intersection of lines joining the Fiducial points.

►To perform computations, one must know:

� H = height above datum from which photos

taken.

� f = focal length of camera lens – either in

in or mm.

►Items on photo:

� Fiducial points

� Date

� Roll and Photo #

Scale of a Vertical Photo

► S = or

► f = focal length 6” or 152.4 mm is common

►H’ = height of plane above ground

► h = height (elevation) of ground

►H = height of place above datum [altimeter

reading (2% error)]

f

H’

f

H-h

Scale of a Vertical Photo

►Datum Scale = the scale which would be effective over entire photo if all points were projected downward to datum.

SD =

► Average Scale = for photo planning

SAV. =

Average elevation can be determined for USGS topo maps, etc.

f

H

f

H-hav.

Relief Displacement

► Relief Displacement exists because photos are a perspective projection.

► Use this to determine the height of object:

h=

h = height of object

d = radial distance to top of object-radial distance to

bottom of object.

r = radial distance to top of object.

d (H’)

r

Planning and Executing Photo Project

► Basic Overall Process:

1. Photography – obtain suitable photos.

2. Control – obtain sufficient control through field

surveys and/or extension by photographic methods.

3. Map Compilation – plotting of planimetric and/or topographic features.

4. Map Completion – map editing and special field surveys.

5. Final Map Drafting

Elements of Planning

1. Conversion of requirements to project specs. � Factors:

1. Purpose of photogrammetry a) Majority of projects for engineering involves making

topographic map in a stereoscopic plotting unit. � Wide angle photography (152mm focal length) is required for

topographic mapping because it provides better vertical accuracy.

� If area is heavily wooded, use f=210mm (standard angle) to allow more visibility through trees.

� Generally 60% overlap with 15-30% sidelap.

� Orientation of flightlines is dictated more by economy than geometric considerations.

Elements of Planning

b) Photos for mosaics should be flown as high as possible. � Reduces relief displacement.

c) Orthophotos – similar to topo maps, however, should be taken normal to ground topo.

2. Photo Scale: somewhat dependent on type of plotter.

� Essentially can be dependent on type of plotter you need to see and dividing it by the resolving power of the photo equipment.

� Also affected by map accuracy and area configuration.

Elements of Planning

3. Allowed scale variation.

� Variation caused by difference in ground elevation and flying height.

� Longer focal length reduces scale variation.

� If flying height remains constant and ground elevation increases the area covered by photo becomes less.

� Overlap becomes less

� Viewfinder needed to control overlap and flightline spacing, thus eliminating possible gaps.

4. Relief displacement

� Affects mosaics most.

� Large amount of relief displacement will make it difficult to form continuous picture desired in mosaics.

Elements of Planning

� Relief displacement decreases as flying height increases, the focal length must also be increased.

� Relief displacement has no adverse affect on map making with stereo.

� With greater relief displacement, elevations can be measured and plotted more accurately.

5. Tilt

� Amount in direction of flight (y tilt). � Will cause overlap to be greater on one end than other.

� Amount normal direction of flight (x tilt).

� Will increase sidelap on one side and decrease on other.

� Y tilt corrected by viewfinder.

� X tilt corrected by increasing planned sidelap.

Elements of Planning

6. Crab and Drift

� Crab – angle formed between flightline and edges of photo in direction of flight and caused by not having focal plane square with direction of flight at time of exposure. � Corrected by rotation of camera on vertical axis through

viewfinder.

� Reduces coverage, but sidelap compensates.

� Drift – plane not staying on flightline. � Most common cause of re-flights and gaps.

Elements of Planning 7. Flying height: determined after sidelap and

overlap determined. � Factors affecting:

1. Desired scale, relief displacement, and tilt.

2. Precision of equipment used.

� Greater precision, greater possible flying height.

� By doubling flying height, ground coverage increased 4 times, thus less ground control and fewer photos.

� Vertical accuracy most important in topographic mapping. 1. Flying height is related to contour interval desired.

� Relationship called C-factor (precision factor)

� Flying height = desired contour interval x C-factor

� C-factor is the value used to compute flying height which will produce photos satisfactory to obtain the desired vertical accuracy of the maps.

Elements of Planning

8. Direction or orientation of terrain � Arrange to fly along ridges, not across.

2. Gathering material and people. 1. Existing photos, maps, survey data, instruments

and personnel.

3. Determine specifications and conditions for operation.

4. Preparing final plans. 1. Scheduling

2. Surveying instructions

5. Cost estimating and replanning.

Flight Design A. Considerations

1. Project boundaries

2. Existing and planned control

3. Time schedule

4. Final product needed

5. Optimum flying season

6. Found cover conditions

B. Objectives

1. Determine optimum conditions for spacing of photos along flightlines.

2. Number and spacing of fligtlines to cover area.

3. Plan must account for allowable deviations.

4. Distance between flightlines on fllightway.

Flight Design

C. Flight Patterns

1. Totally dependent on overlap and sidelap.

� Under ideal conditions with 9”x 9” photo with 6” focal length, and overlap of 57%, and sidelap of 13% will provide maximum stereo coverage with no gaps.

� If additional safety factor desired, overlap can be increased to 70-75% and sidelap can be increased to 50%.

Computation of Flight Plan ► Data required to compute flight map lines, time

interval between exposures, and amount of film

needed.

1. Focal length of camera.

2. Flying height above datum or photo scale for certain elevation.

3. Size of photo.

4. Size of area to be photographed.

5. Positions of outer flight lines with respect to boundary.

6. Overlap.

7. Sidelap.

8. Scale of flight map.

9. Ground speed of aircraft.

Example

Area – 15 miles N-S & 8.5 miles E-W

Photos – 9” x 9”

Save tobe 1:12000 @ 700’ above elevation

Overlap – 60%

Sidelap – 35%

Ground speed of plane – 150 mph

Flight lines to be laid out N-S on a map @ a scale

of 1:62500

Outer flight lines coincide with E & W boundary

1. Flying Height:

12000’ above 700’ or 12700’ above sea level

2. Ground Distance Between Flight lines – since sidelap is 35%, photo distance between lines is 65% of 9”=5.85”

3. Number of flight lines

Total width = 8.5 miles x 5280 = 44880’

flight lines (Round up)

4. Adjust ground distance between flight lines

5. Spacing of flight lines on flight map

5610’ on map @ 1:62500 scale

'5850'1/"12

12000'85.5=

×=∴ ingGroundSpac

=∴= HH 12000

11

9185850

44880=+=∴

'561019

44880=

−

"08.1'1

"12

62500

5610=×

6. Ground Distance Between Exposures with 60% overlap gain on

each photo is 40%

40% of 9” = 3.60” ground distance is:

∴

∴ '3600'1/"12

1200060.3=

×