Photogrammetry

1. Photogrammetry

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 cannot be used successfully over the following types of terrain. • Deserts or plains, 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 inches 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)]

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.

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.

18. Planning and Executing Photo Project • Basic Overall Process: • Photography – obtain suitable photos. • Control – obtain sufficient control through field surveys and/or extension by photographic methods. • Map Compilation – plotting of planimetric and/or topographic features. • Map Completion – map editing and special field surveys. • Final Map Drafting

19. Elements of Planning • Conversion of requirements to project specs. • Factors: • Purpose of photogrammetry • 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 • Photos for mosaics should be flown as high as possible. • Reduces relief displacement. • Orthophotos – similar to topo maps, however, should be taken normal to ground topo. • 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 • 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. • 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. • 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 • 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 • Flying height: determined after sidelap and overlap determined. • Factors affecting: • Desired scale, relief displacement, and tilt. • 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. • 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 • Direction or orientation of terrain • Arrange to fly along ridges, not across. • Gathering material and people. • Existing photos, maps, survey data, instruments and personnel. • Determine specifications and conditions for operation. • Preparing final plans. • Scheduling • Surveying instructions • Cost estimating and replanning.

26. Flight Design • Considerations • Project boundaries • Existing and planned control • Time schedule • Final product needed • Optimum flying season • Found cover conditions • Objectives • Determine optimum conditions for spacing of photos along flightlines. • Number and spacing of fligtlines to cover area. • Plan must account for allowable deviations. • Distance between flightlines on fllightway.

27. Flight Design • Flight Patterns • 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. • Focal length of camera. • Flying height above datum or photo scale for certain elevation. • Size of photo. • Size of area to be photographed. • Positions of outer flight lines with respect to boundary. • Overlap. • Sidelap. • Scale of flight map. • 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. Flying Height: 12000’ above 700’ or 12700’ above sea level • Ground Distance Between Flight lines – since sidelap is 35%, photo distance between lines is 65% of 9”=5.85” • Number of flight lines Total width = 8.5 miles x 5280 = 44880’ flight lines (Round up) • Adjust ground distance between flight lines • Spacing of flight lines on flight map 5610’ on map @ 1:62500 scale

31. Ground Distance Between Exposures with 60% overlap gain on each photo is 40% 40% of 9” = 3.60” ground distance is: