Measuring Features on Aerial Photographs Determining scale and

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

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

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

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

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

Calculate using H and f n n n PS = f / H 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

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

Estimating area measurements

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

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

Shadow length n n n Object height = shadow length * Tan (sun angle) The length of a shadow is directly related to the features height Need to know the solar elevation angle n n Determine from solar tables 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

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

Measure parallax n n n Based on the difference in parallax (radial displacement) between the top and bottom of an object More accurate than single photo methods 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 stereo photos with minimum tilt

Parallax equations n h = (H * d. P) / (PR + d. P) n n h = height of object H = Flying height PR = Stereoscopic parallax of the reference plane PR = PP 1 PP 2 - f 1 f 2 d. P = Difference in stereoscopic parallax between the top and bottom of the object d. P = f 1 f 2 – t 1 t 2