Lab 7

Goals and Background

The purpose of this lab was to learn to perform photogrammetric tasks on aerial photographs including

1.  Calculating scales, measurements, and relief displacements.
2.  Using stereoscopy for analysis
3.  Performing orthorectification

Methods and Procedures

To determine the scale of the Photograph

1.  I measured the size of an object on the computer screen and divided that by its real world height which gave me the scale for the aerial photograph

 

A and B marked on the image

The distance from A to B in the image is 2.75 inches

The distance in the real world is 8822.47 feet

8822.47 multiplied by 12 equals 105869.64 inches

105869.64 divided by 2.75 is approximately 38498

The scale of the aerial photograph is 1 to 38498

2.  For a different aerial photograph I used an equation to determine the scale as shown below

Equation: S= f/(H-h)

S= scale

F= focal lens length

H= altitude above sea level

h= elevation of terrain

F= 152 mm

H= 6096000 mm

h= 242620.8 mm

S= 152 / (6096000-242620.8) 

S= 152 / 5853379.2

 

To measure features on aerial photographs

1.  Use the Measure Polygon digitizing tool in Erdas to draw a polygon around the selected feature and double click when completed

 

To calculate relief displacement

Take the necessary measurements and use the formula below

d= (h*r)/H  

d=relief displacement

h= (.25x3209)  inches

r= (5.5x3209) inches

H= (3980x12) inches

d= (802.25*17649.5)/47760

d= 296.47 inches

For Orthorectification

Orthorectification involved setting many properties that would be lengthy to explain individually.  The main steps that were taken however, is that control and Tie points were selected in order to orthorectify the image.  The purpose of orthorectification is to produce an image that is spatially correct, with relief displacement and other geometric errors removed.

Results and Reflections

The final orthorectified image can be seen in the image below

Orthorectified Image

 

Although the orthorectification process was very time consuming, the process made much more sense after having gone through it.  Overall I think the lab helped improve my skills in photogrammetry. 

 

Sources

(NAIP) images: 
United States Department of Agriculture, 2005.
(DEM) for Eau Claire, WI:

United States Department of

Agriculture Natural Resources Conservation Service, 2010.
Spot satellite images:
Erdas Imagine, 2009.