Lab 7: Photogrammetry

Introduction and Background:

The main goal of this lab was to develop photogrammetric shills and to understand the mathematics behind different photographic scales, perimeters, area, and relief displacement. Furthermore, this lab was designed to make sure participants preform orthorectification of overlapping satellite images.

Photographic scales is the relationship between two different features within an aerial photograph verses the actual distance between the same features on the ground. While orthorectification is the process of correcting positional and elevation errors associated with overlapping aerial photos, in order to create a seamless flow between them.

There were three parts to this lab, the first revolved around photographic scales and displacement. The second was based in stereoscopy (note: This portion of the assignment will not be discussed within this write up.), and the third was meant for the process of orthorectification. The goals associated with the parts in discussion are as follows:

• Part 1: To figure out photographic scale, and relief displacement of an aerial photograph, and also measure the perimeter and area of a feature within that same image.

• Part 3: To use Erdas IMAGINE to orthorectify overlapping aerial photographs in order to create a seamless picture of the area

Methods:

Before any work could be preformed, data management needed to be done. This involved the development of an active work folder where all data, images, and outputted work could be stored, managed, and organized.

Below are the steps associated with part one, and part three.

Part 1: Scales and Relief Displacement

Figure 1: EauClaire_west_se.img

All of the calculations made within this part of the lab was done in relation to the EauClaire_west_se.jpg image (Figure 1) that was provided for us and located within the lab_7 folder that was previously made during data management.

Figure 2: Aerial Scale

The first calculation conducted in relation to this image was finding the photo's aerial scale. to do this, the equation featured in figure 2 was used. In order to find a measurement for "pd" (photograph's distance) a ruler was used to measure the distance in of the red line featured in figure one.

• "pd" = 2.75 in 
• "gd" = 8822.471 ft (105869.64 in)

This number was then divided by the "gd" (ground distance) to find the photograph's scale. However, both scales need to use the same unit. So before this was done, the "gd" was converted into inches.

Figure 3: Photographic Scale

The next scale that was collected was done by using the equation featured in figure 3.

• "f" (diameter of focal lens) = 152mm (0.497 ft) 
• "H" (altitude) = 20,000 ft
• "h" (elevation above Sea Level) = 796 ft

Feature 5: Relief Displacement

 The following calculation was done using Erdas IMAGINE. Within this program, perimeter and area was calculated for the water feature marked by the letter "A" in figure 4, using the measurement polygon tool. In order to do this, the water feature's shoreline was outlined, or digitized. Area was recorded both hectares and acres, and perimeter was recorded in meters and miles.

Figure 4: Water feature digitized 

The final calculation (figure 5) that was calibrated in order to find the relief displacement of the smokestack in figure 6. Again a ruler was used to measure the height and radial distance (the distance from the smokestack to the principle point) of the smokestack in figure 6. To find "h" the image's height was multiplied by the scale. The equation then contained:

• "h" (real world height) = (0.5in)(3,209) = 1604.5 in 
• "r" (radial distance) = 10.5 in
• "H" (aerial camera's height) = 3,980 ft = 47760 in

Figure 6: The smokestack that was measures is can be found by the letter "A" and by its associated shadow.

Part 3: Orthorectification

While using Erdas IMAGE, the photogrammetry tool was selected to start the orthorectification process. Within this tool, a model set up dialog box appeared. In this box, the catagory was changed to polynomial-based Pushbroom and then, SPOT Pushbroom. After this, the horizontal reference coordinate system was set to the desired projection type.

The next step was to add in the spot_pan.img into the program. All default were accepted for the image's frame properties. From here, the point measurement tool was accessed in order to start collecting ground control points (GCP's). In order to collect GCP's a reference image (xs_ortho.img) needed to also be added into the program. Figure 7 shows the reference image (left) and the imaged being rectified (right). 

Figure 7: Split view of xs_ortho.img (left) and the spot_pan.img (right)

Once both of these steps were done, GCP's  could then be collected. To do so, a reference GCP must be collected on the left image, then the corresponding spot needed to be located and recorded on the right image. This was done for 11 separate GCP's. Figure 8 shows the location of the 11 GCP's in relation to the two images.

Figure 7: The locations of the 11 GCP's located.

At this point in time, the elevation information was set by using the palm_springs_dem.img. Then the point number column associated with the GCP's was changed to the "value" and the the type column changed to "full"

GCP's were then collected only within the area of the overlapping images. This was accomplished first bt adding in the spot_panb.img into the program. The set up process was the same as the spot_pan.img and the reference GCP's remained fixed. The new GCP's collected were only in reference to the overlapping portions of the images. To collect more GCP's, the automatic tie point tool was used. This tool added in a total of 40 GCP's  on the two images. Once this was done, there was enough GCP's to run the triangulation tool. However before this was ran, the standard deviation method used to do so was set to a weighted value of 15 for all x, y, z coordinates. The final step was then to run the ortho resampling tool. Again before this was ran some changes needed to be changed. This included making sure the resampling method was set to billinear interpolation, and that both of the spot_pan.img and spot_panb.img were added into the dialog box. 

The final images were then viewed in Erdas IMAGE.

Results:

Results are as follows:

Part 1:

• Aerial Scale = 1:38,4981.1
• Photographic Scale = 1: 2.588
• Area: 37.82 ha or 92.88 acres
• Perimeter: 4099.33 meters or 2.54 miles
• Relief Displacement = 0.35 in

To fix displacement issue, the image needs to be shifted 0.35 in towards the principle point.

Part 3:

Figure 9 in the the two images that were orthorectified to create a seamless image, while figure 10 depicts the accuracy shown at the border line. 

Figure 9: spot_pan.img and spot_panb.img after rectification

Figure 10: The boundary line of the two images is shown by the black line. 

Sources:

National Agriculture Imagery Program, United States Department of

Agriculture. (2005).

. 

Digital Elevation Model (DEM) for Eau Claire, WI, United States Department of

Agriculture Natural Resources Conservation Service. (2010). 

Lidar-derived surface model (DSM) for sections of Eau Claire and Chippewa, Eau Claire County and Chippewa County. 

Spot satellite images, Erdas Imagine. (2009).

Digital elevation model (DEM) for Palm Spring, CA, National Aerial Photography Program.

(NAPP) 2 meter images, Erdas Imagine. (2009).