Lab 7: Photogrammetry

Goal:

The main objective of this lab was to develop skills of performing photogrammetric tasks on aerial photographs and satellite images. More specifically, this lab was to help understand the math behind the calculation of photographic scales, measurement of areas and perimeters of features, calculating relief displacement, and introduce stereoscopy and performing orthorectification on satellite images.

Methods

The first section of the lab was to calculate scale of a photo when measuring the distance between two objects on the screen and using that distance and the distance in real life to come up with a scale.  This resulted in a scale of 1/39,393.  The distance measured was from point A to point B.

The next thing done was to measure perimeters and areas from a image.  The measure perimeters and areas tool was used in Erdas to measure the area and perimeter of the lake in the middle left portion of the image above.  The pond has a area of 94.5 Acres and a perimeter of 2.54 Miles.

The next section was to calculate relief displacement from object height from an image. The camera height and scale of the image were known.  The relief displacement was 4.58 in.

The next part of the lab was to generate a 3D image using an elevation model.  To do this the Terrain-Anaglyph tool was used in Erdas to combine a digital elevation model with an image of the study area with a 1 meter spatial resolution. This image below is 3d when viewed with Polaroid glasses.  

This same process was done with a digital surface model.  The result is seen below. 

The last part of the lab was to orthorectify images and create a planimetrically true othoimage. Erdas Imagine Lecia Photogrammetric Suits (LPS) was used to perform this task.  A new block file was created in LPS and certain parameters were set.  This geometric model interface was set to polynomial-bases pushbroom and the geometric model catagory was set to SPOT pushbroom.  After this the coordinate system and projection type were chosen along with other parameters.  This puts the images that are going to be used in the process to the same coordinate system and projection.  

The next step was to bring in satellite imagery of Palm Spring, CA (spot_pan.img) and add it to the block in LPS.  No changes were done to the image but the verification process was completed to specify the sensor.  Next, ground control points were collected using the Classic Point Measurement Tool in LPS using a reference image.  The corresponding points were collected on the spot_pan.img using the point measurement tool.  Using the automatic drive function, the remaining GCP's were collected. The last two points were collected from a different image with a different horizontal reference source.  Elevation information was collected from a DEM file using the Reset Vertical Reference Source button in the point measurement tool palette. 

The next step was to bring in another image (spot_panb) and GCP's were collected for this image.  The GCP's for this image were collected using the already rectified spot_pan image.  Tie points were then collected for both images using Automatic Tie Point Generation Properties in the Point Measurement Tool Palette.  Parameters were set and the accuracy of the tie points were confirmed to make sure the image was properly orthorectified. 

LPS project manager was then used to perform Triangulation.  Parameters were set and the resulting text file was saved.

The final orthorectified image was created by using the Start Ortho Resampling Process in LPS.  Parameters were set and images were created.  The images were then brought in and overlapped to compare the spatial accuracy of the images.  Seen Below.

Results:

Anaglyph using DEM

Anaglyph using DSM

Orthorectifed Images

Conclusion:

When using satellite images, scale, perimeter, area, and relief displacement can be calculated and used for further analysis.  Anaglyphs can be created from elevation and surface models to make 3D models that would be useful in many situations.  Lastly, Erdas makes orthorectifying images relatively simple.  This makes a planimetrically correct image that has a constant scale that represents objects where they truly are. 

Sources:

Hexagon Geospatial. (2009). Erdas Imagine [computer software]. Georgia: Norcross. 

United States Department of Agriculture. (2005). [Satellite images in img. format]. National Agriculutre Imagery Program. Retrieved from: https://gdg.sc.egov.usda.gov/.

United States Department of Agriculture Natural Resources Conservation Service. (2010). [Digital elevation model for Eau Claire in dbf. format]. Retrieved from: http://www.nrcs.usda.gov/wps/portal/nrcs/site/national/home/.