The goal of this lab was to become familiar with stereoscopy and orthorectification tasks on satellite and aerial images. These tasks were accomplished understanding the mathematical models used to calculate photographic scales, the measurement of areas and perimeters of features, and the calculating of relief displacement.
Methods:
PART 1 - Scales, Measurements, and Relief Displacement
Section 1: Calculating Scale of Nearly Vertical Aerial Photographs
I was given two different, nearly vertical images and my first task was to determine the distance between two points which I would then use to calculate the scale. I was given the ground distance of 8824.47ft between point A and point B. I then measured the distance between the two points on an aerial image of the AOI on my screen and determined the scale through:
2.7 inches on the photo, 8824.47ft in real world
2.7in/(8824.47ft*12in)
2.7in=105,869.64in
1in=39,210.9778in
Scale = 1:40,000
I was then given an image taken by a high altitude reconnaissance aircraft of Eau Claire County. The photograph was acquired at an altitude of 20,000 feet above sea level and had a camera focal length lens of 152mm. I was told that the elevation of Eau Claire County is 796ft above sea level. I then determined the scale of this photograph with the following equation:
S = f/(H/h)
f = 152mm
H = 20,000ft
h = 796
S = 152mm/(20,000ft - 796ft)
S = 5.98in/(240,000in - 9552in)
S = 1in/38536.45in
Scale = 1:39,000
Section 2: Measurement of Areas of Features on Aerial Photographs
I used the 'Measure Perimeters and Areas' digitizing tool to draw a polygon around a feature (pond) on the image. When I finished digitizing, I was given the feature's perimeter and area in different units.
Section 3: Calculating Relief Displacement From Object Height
Here I had to figure out the relief displacement of a smoke stack in an aerial image. I was given the height of the aerial camera above the datum (3980ft), the scale of the aerial photograph (1:3209), and I measured the distance from the principal point to the object, which I used to mathematically figure out the relief displacement.
PART 2 - Stereoscopy
This part of the lab focused on creating and analyzing an anaglyph image. I brought in an image to Erdas with a 1 meter spatial resolution and a DEM with a 10 meter spatial resolution. I then used the 'Anaglyph' tool to input the images. I increased the vertical exaggeration to 2 and ran the model which created a new anaglyph image which I was able to analyze with the use of polaroid glasses.
PART 3 - Orthorectification
The goal of this part of the lab was to become familiar with Erdas Imagine Lecia Photogrammetric Suite (LPS), which is used for triangulation, orthorectification of images collected by numerous sensors, etc. With the use of LPS, images could be orthorectified and in the process create a planimetrically true orthoimage.
Section 1: Create a New Project
I created a new project using an image of Palm Springs, California by opening the Imagine Photogrammetry Project Manager to create a new block file. I used a Polynomial-based Pushbroom and SPOT Pushbroom in my Geometric Model Category. I set the projection type to UTM, the spheroid to Clarke 1866, and the datum to Nad27 (Conus) with the UTM Zone 11.
Section 2: Add Imagery to the Block and Define Sensor Model
Now it was time to input the images to the block and define the sensor model. I added the images and verified the parameters of the SPOT pushbroom sensor
Section 3: Activate Point Measurement Tool and Collect GCPs
I selected 'Start Measurement' and used the 'Classic Point Measurement Tool'. I then inputted both images using the orthorectified image as a reference to correct the second image. I then collected 2 GCPs where I selected the "Automatic (x,y) Drive' icon after they were collected. I then collected seven more GCPs until I had a toltal of nine. After the ninth GCP, I reset my horizontal reference sources to a different image until I had a total of 11 GCPs. Next, I chose the 'Reset Vertical Reference Source' icon and chose DEM to set the DEM to my reference image file. I then clicked on the 'Update Z Values on Selected Points' icon to obtain the elevation information for the GCPs.
Section 4: Set Type and Usage, Add a 2nd Image to the Block and Collect its GCPs
After I obtained all of the elevation information, I was finished collecting GCPs for the first image. So I added a second image to the block and collected 11 more GCPs in the same manner as for the first image.
Section 5: Automatic Tie Point Collection, Triangulation, and Ortho Resample
Here I used the 'Automatic Tie Point Generation Properties' icon from the 'Point Measurement' tools and set the image to 'all available' and the initial type to 'Exterior/Header/GCP'. I then set the 'Intended Number of Points/Image' to 40. After the tie points were created, I looked at the tie point summary to see how accurate my GCPs were. Next I clicked 'Edit-Triangulation Properties' and changed the 'Iterations of Relaxation' value to 3, to start triangulation. I verified and changed parameters to 'Same Weighted Values' and X, Y, and Z fields to 15. I then ran the model.
I used the 'Start Ortho Resampling Process' icon and set the 'DTM Source' to DEM and input my DEM image. I selected 10 as the output x/y values and verified that the resampling technique was 'Bilinear Interpolation'. I then clicked the Add button and I input my second image. I then ran the model.
Section 6: Viewing the Orthorectified Images
I viewed both of my images into Erdas Imagine to see how successful the image correction was.
Results:
Fig. 1: This image was used in Part 1 section 1 to determine the scale of the aerial photograph.
Fig. 2: This image was used to determine the relief displacement of the smoke stack.
Fig. 3: This figure shows the final orthorectification image
Sources:
Erdas Imagine, 2009. Digital Elevation Model (DEM) of Palm Springs, CA.
Erdas Imagine, 2009. National Aerial Photography Program (NAPP) 2 meter images.
Erdas Imagine, 2009. Spot satellite images.
United States Department of Agriculture, 2005. National Agriculture Imagery Program (NAIP).
United States Department of Agriculture and Natural Resources Conservation Service, 2010. Digital
Elevation Model (DEM) of Eau Claire, WI.
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