1. Measuring Features on Aerial Photographs Determining scale and measuring areas and height

2. Determining photo scale Compare with maps Compare with maps Compare with known features on the ground Compare with known features on the ground Calculate using flying height and focal length Calculate using flying height and focal length Estimate based on experience Estimate based on experience

3. Some variables PS = Photo scale (unitless) PS = Photo scale (unitless) RF = Representative fraction RF = Representative fraction PD = Photo distance PD = Photo distance GD = Ground distance GD = Ground distance MD = Map distance MD = Map distance F = focal length F = focal length H = Height of camera above ground H = Height of camera above ground PSR = Photo scale reciprocal (unitless) PSR = Photo scale reciprocal (unitless) MSR = Map scale reciprocal (unitless) MSR = Map scale reciprocal (unitless)Notes: Units must be the same Units must be the same Photo scale is variable if photo is tilted or terrain is not flat Photo scale is variable if photo is tilted or terrain is not flat

4. Compare with maps of known scale RF = 1 / [MD*MSR)/PD)] RF = 1 / [MD*MSR)/PD)] Best if: Best if: two lines are measured on the photo two lines are measured on the photo intersecting close to right angles intersecting close to right angles near the principal point near the principal point as long as possible as long as possible terminating at points with the same elevation terminating at points with the same elevation

5. Compare with known features PS = PD / GD PS = PD / GD Identify features with know dimensions such as a football field Identify features with know dimensions such as a football field Identify features that can be easily measured Identify features that can be easily measured Best to use the same conditions that you would use if comparing with map distance Best to use the same conditions that you would use if comparing with map distance

6. Calculate using H and f PS = f / H PS = f / H Scale applies to an imaginary plane on the ground at a distance H from the camera Scale applies to an imaginary plane on the ground at a distance H from the camera At low altitudes a small error in H or focal length results in a significant error for PS At low altitudes a small error in H or focal length results in a significant error for PS

7. Estimate based on experience With experience photo interpreters can estimate the scale based on the size of features in the photo With experience photo interpreters can estimate the scale based on the size of features in the photo

8. Estimating area measurements

9. Directly measuring area Planimeter digital or analog) Digitizing tablet On screen digitizing

10. Measuring height Single photo Single photo Shadow length Shadow length Measure extent of displacement Measure extent of displacement Stereo photos Stereo photos Measure parallax Measure parallax

11. Shadow length Object height = shadow length * Tan (sun angle) Object height = shadow length * Tan (sun angle) The length of a shadow is directly related to the features height The length of a shadow is directly related to the features height Need to know the solar elevation angle Need to know the solar elevation angle Determine from solar tables Determine from solar tables Calculate from shadow length cast from an object of known height Calculate from shadow length cast from an object of known height Figure from Remote Sensing Core Curriculum Introduction to Photo Interpretation and Photogrammetry by John Estes and Jeff Hemphill

12. Extent of displacement Height = (D * H) / R Height = (D * H) / R D = displacement length of object D = displacement length of object R = photo distance from nadir to top of object R = photo distance from nadir to top of object H = Flying height H = Flying height Best with large photo scales Best with large photo scales

13. Measure parallax Based on the difference in parallax (radial displacement) between the top and bottom of an object Based on the difference in parallax (radial displacement) between the top and bottom of an object More accurate than single photo methods More accurate than single photo methods Can be used even if the foot of the tree isn’t visible (use reference with same elevation) Can be used even if the foot of the tree isn’t visible (use reference with same elevation) Requires more skill and special tool (parallax bar or parallax wedge Requires more skill and special tool (parallax bar or parallax wedge Requires stereo photos with minimum tilt Requires stereo photos with minimum tilt

14. Parallax equations h = (H * dP) / (P R + dP) h = (H * dP) / (P R + dP) h = height of object h = height of object H = Flying height H = Flying height P R = Stereoscopic parallax of the reference plane P R = PP1PP2 - f1f2 P R = Stereoscopic parallax of the reference plane P R = PP1PP2 - f1f2 dP = Difference in stereoscopic parallax between the top and bottom of the object dP = f1f2 – t1t2 dP = Difference in stereoscopic parallax between the top and bottom of the object dP = f1f2 – t1t2