LAB 2 - PHOTOGRAMMETRY

(revised 06/01)

The purpose of this lab is to make you aware of some of the basic concepts and terminology of analytic photogrammetry. The problems you will solve are relatively simple and only scratch the surface of photogrammetric techniques, they are intended to give you hands-on experience in making measurements and reinforce material presented in the text.

TERMINOLOGY

ENDLAP: The overlap of aerial or space photographs or images along the flightline or track of the platform. (Normally 55-65% for stereoscopic coverage).

SIDELAP: The area common to two photos in adjacent flight strips; the amount is expressed as a percentage of the total photo area. (Normally 20-25% for stereoscopic coverage).

DRIFT: The horizontal displacement of the aircraft, caused by the wind, from the track it would have followed in still air.

CRAB: Any turning of an airplane which causes its longitudinal axis to vary from the track of the plane.

PRINCIPAL POINT (pp): This is the optical center of a photograph. The point at which the optical axis of the camera (line passing through the center of the lens) intersects with the ground datum plane (theoretical surface of the area covered on the photograph).

FIDUCIAL MARKS: Index marks (usually 4) found on most optical imagery. The marks are adjusted so that the intersection of lines drawn between opposite fiducial marks defines the principal point.

NADIR: That point on the ground vertically beneath the perspective center of the camera lens. (The nadir = the principal point on a perfectly vertical photograph).

CONJUGATE PRINCIPAL POINT (cpp): The location on the photograph that corresponds with the pp of the adjacent print in the same flight line.

PHOTOBASE: The distance between the principal points of two adjacent prints of a series of vertical aerial photographs. It is usually measured on one print after transferring the principal point of the other print.

FLIGHTBASE: The distance on the ground between each principal point.

RELIEF DISPLACEMENT: The difference in the position of a point above or below the datum, with respect to the datum position of that point, owing to the perspective of an aerial photograph.

PARALLAX: The apparent change in the position of one object, or point, with respect to another, when viewed from different angles. Parallax is a normal characteristic of overlapping aerial photographs; it is the basis of 3-dimensional viewing.

After you have completed this lab exercise you should be able to calculate endlap, sidelap, draft & crab, and the height of objects by both the displacement and parallax methods.

For questions 1 through 4, each student uses 1(one) of the available groups of photos. Indicate which strip was used. You only use one strip; not all of them.

1a) Calculate endlap between each photo.

1b) Calculate average endlap for the 4 photos of the strip.

2) Calculate drift and crab for the 4 photos of the strip.

3) Find the average photobase and flightbase of the strip

(photobase in inches and flightbase in feet).

4) On an overlay draw the flight line of the strip. Show photo edges and pp and cpp.

5) The top of a water tower is displaced 3mm. If the top of this tower is 3.26 inches from the principal point, and the flight altitude is 8000 feet, how high is the tower (nearest foot)?

6) A vertical stereopair has an RF of 1:12,000 and a focal length of 6 inches. If the radial displacement is 0.4 feet from the center of the photo to the top of the building that is 120m high what is the amount of displacement on the photo in mm?

7) The photobase measurement for a vertical stereopair is 0.25 feet. The height of a silo is 300m. The distance between the tops of the silos on the photos is 0.25 feet, and the distance between the bases of the silos is 79.25 mm. What is the altitude of the plane at the time the photo was taken (nearest foot)?

8) A monument is shown on a vertical stereopair which has a scale of 1:2500 and F.L. of 152.4mm. The distance between the pp and cpp on one photo is 3.125 inches. The distance between the cpp and pp on the other photo is 3.5 inches. The distance between the bases of the monuments is 6.75 cm. How tall is the monument if the distance between the tops of the monuments on the photo is 6.63cm (nearest foot)?

9) The shadow of a tree is 0.5 inches. The shadow of another tree on the photo is 0.16 inches. If the first tree is 100 feet high, how tall is the other tree (nearest foot)?

10) Photo: Stuyvesant Plaza

Find the height of the office building in the lower left corner to the nearest foot using the displacement method.

11) Photo: Troy

Find the height of the flag pole circled in the lower right-hand corner to the nearest foot using the displacement method.

12) If these photos were taken at an altitude of 220 feet above sea level, what is the height of the monument to the nearest foot?

*BONUS: Why will this particular measurement produce inaccurate results?