Photogrammetry ppt

1. Photogrammetry
CE 426

2. Introduction
►Definition of Photogrammetry: the art,
science, and technology of obtaining
information about physical objects and the
environment by photographic and
electromagnetic images.

3. 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.

4. 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.

5. 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.

6. 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.

7. 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.

8. 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.

9. 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.

10. 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.

11. 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.

12. 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.

13. 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.

14. ►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 #

15. 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

16. 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.

17. 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

18. 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

19. 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.

20. 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.

21. 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.

22. 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.

23. 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.

24. 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.

25. 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.

26. 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.

27. 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%.

28. 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.

29. 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

30. 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
 H
H 12000
11
918
5850
44880

'5610
19
44880


"08.1
'1
"12
62500
5610


31. 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

