Lab 7: Photogrammetry

Goal and Background

The goal of this lab was to improve our understanding of the math behind scales, measurement of areas and perimeters, and calculating relief displacement. It also serves as an introduction to stereoscopy and orthorectification.

Part 1: Scales, measurements and relief displacement

Section 1

The first part of the lab involved finding the scale of an aerial image. Given a section of highway that we already knew the real world distance for, we were able to find the scale of the image by measuring the distance from one point to the other on the image itself.Next we found the scale of a photo when only knowing the altitude of the aircraft carrying the camera, the focal length of the camera, and the elevation of Eau Claire County.

Section 2

Figure 1 - Digitized area to be measured

Section 2 involved finding the area and perimeter of features in an aerial photo of Eau Claire. I opened the 'Measure' tool and selected 'Measure Perimeters and Areas'. This allowed me to digitize an area and find out what the area or perimeter was. After digitizing, I was able to change the units of measurement and have the results update on the fly.

 Section 3

Figure 2 - Image of Eau Claire area used for calculating relief
displacement

For section 3, we calculated relief displacement. Figure 2 shows the image and feature that was used for this exercise.

Knowing the height of the aerial camera above datum (3,980 ft), the scale of the image (1:3,209), and by finding the real world height of the smoke stack (by using the image scale), we can find the relief displacement of the smoke stack labeled 'A' in the image.

Part 2: Stereoscopy

In part 2 of the lab, we learned how to create a stereoscopic image of Eau Claire. First, I opened an image of Eau Claire at 1 meter spatial resolution and a DEM (digital elevation model) of the same area at 10 meter spatial resolution. Under 'Terrain' I chose 'Anaglyph' to open 'Anaglyph Generation', the tool I would be using. I brought in my two images and set the vertical exaggeration to 2. 

When the tool finished running, I had a 3-D image of Eau Claire needing polaroid glasses in order to view it. The result was impressive, though slightly too exaggerated in some places. It did, however, make it easier to interpret geographical features in Eau Claire.

Part 3: Orthorectification

Figure 3 - LPS Project Manager window

Satellite and aerial images usually have many geometric errors that must be corrected before they can be used professionally. Orthorectification is the process of using currently orthorectified photos to correct new ones.

LPS Project Manager (Figure 3) is the tool used for orthorectification. This is found under the "Toolbox" tab.

Figure 5 - GCP collection

By setting a projection, and adding two overlapping images, we can orthorectify (figure 4). Similar to geometric correction, we used GCPs (ground control points) on both images to match the locations. Figures 4 and 5 illustrate this process.

Figure 4

The final images are perfectly lines up at the edges. Figure 6 shows how effective the process of Orthorectification is. This process is normal done by the company or group collecting the images, so this is completed already when the final image gets to public use.

Figure 6 - Orthorectified image.