Lab 7: Photogrammetry

Photogrammetry

Goal

The purpose of this lab was to become acquainted with using tools to turn distorted aerial and satellite images into photogrammetrically correct images. Mathematics including measurements of areas and perimeters, as well as relief displacement will be used. Stereoscopy and orthorectification will also be introduced.

Methods

Part 1: Scales, Measurements, and Relief Displacements

Section 1: Calculating scale of nearly vertical aerial photographs

The image below was used to practice scale calculation. With a known ground distance (8822.47 ft), the photo distance from A to B can be measured with a ruler to obtain the scale of the photo. The equation looks like:

2.625”/8822.47’ = 2.625”/105,869.64” = 1/40,331.29

S = 1 : 40,331”

Figure 1: Image with known ground distance and photo distance measured with ruler.

Scale can also be determined using flying altitude above sea level, elevation of land above sea level, and focal lens length. The scale of the image below was found by using the following equation:

152 mm/ (20,000-796) ft = 152 mm/19,204 ft = 152/5853379.2 = 1/38509

S = 1 : 38,509 mm

Figure 2: Image with known altitude above sea level, elevation of land above sea level, and focal lens length.

Section 2: Measurement of areas on aerial photographs

Sometimes, the areas and perimeters of features must be measured. ERDAS Imagine make this easy by calculating these from polygons drawn on images, with the tools highlighted and measurements recorded as shown in Figure 3. 

Figure 3: Area and perimeter calculation using the measurement tool in ERDAS Imagine

Section 3: Calculating relief displacement from object height

Relief displacement is caused by distance from the principle point of an image. The relief displacement of the image below was calculated using the following equation: 

0.5” of displacement

Real world height = 3209 x 0.5 = 1604.5”

10.4375” away from PP

RD = (1604.5 x 10.4375) / 3980 = 4.21”

Figure 4: Relief displacement of smokestack (labeled "A"). 

Part 2: Stereoscopy

Section 1: Creation of anaglyph image with the use of a digital elevation model (DEM)

Anaglyphs allow viewers of an image to see a 3 dimensional images with the help of polaroid glasses. One way of achieving this is by using a DEM along with a 2 dimensional photograph. The Anaglyph tool in the Terrain window was used. The specifications below were used in the dialogue box before running the model:

Figure 5: Specifications for DEM anaglyph

Section 2: Creation of anaglyph image with the use of a LiDAR derived surface model (DSM)

Another way of achieving this is by using a DEM along with a 2 dimensional photograph. The Anaglyph tool in the Terrain window was used again. The specifications below were used in the dialogue box before running the model:

Figure 6: Specifications for LiDAR anaglyph

Part 3: Orthorectification

Section 1: Create a new project

The images below were taken by the SPOT satellite and require orthorectification.

Figure 7: SPOT satellite images in need of orthorectification

From the help menu, a search was done for "photogrammetry" and "Photogrammetric Project" was selected to begin a new block file project.

"Polynomial-based pushbroom" and "SPOT Pushbroom" were selected from the dropdown menu for Geometric Model Category.

In the Block Property Setup, the Horizontal Reference Coordinate System was set to the following specifications:

Projection Type: UTM.

Spheroid Name: Clarke 1866.

Datum Name: NAD27(CONUS).

UTM Zone field: 11.

North or South Field:  North.

Axis Order: E,N.

Horizontal Units: Meters.

Section 2: Add imagery to the block and define sensor model

The top image in Figure 7 was added to the block file by selecting "Add".

The parameters of the satellite were specified as "SPOT PAN" in "Interior Orientation". Specifications are shown below:

Figure 8: Specifications for Interior Orientation

Section 3: Activate point measurement tool and collect GCP's

"Classic Point Measurement" was selected from the "Point Measurement Tool." With the point measurement window opened, as shown below, the reference layer was set to a previously orthorectified image and the checkbox for "Use Viewer as a Reference" was clicked. 

Figure 9: Point measurement window with orthorectified image set as reference layer (left)

At this point GCP's could begin to be collected. The Select Point tool was used to move the inquire box as appropriate. The "Add" button was selected for point 1 on the reference image and subsequently for point 1 on the distorted image. X and Y reference coordinates as well as X and Y file coordinates were provided by the lab to ensure accuracy. This process was completed for 9 points.

Another horizontal reference source was brought in for the last 2 points by selecting "Reset Horizontal Reference Source." The same GCP selection process was used for this source for points 11 and 12.

The Point Measurement was saved.

The Vertical Reference Source dialogue was opened and set to a DEM of Palm Springs. All the previously gathered points were highlighted and their Z values were updated to the reference DEM, as shown below.

Figure 10: Z values updated from DEM of Palm Springs

Section 4: Set Type and Usage, add a 2nd image to the block & collect its GCP's

The Type and Usage columns (shown also in Figure 10) was right clicked for each point and updated to "Full" and "Control", respectively, shown below. The Point Measurements were then saved and closed.

Figure 11: Type and Usage columns updated to "Full" and "Control"

The same GCP collection process was done for the second image to be orthorectified shown in Figure 7 by clicking "Add Images" in the contents window. Interior Orientation was set the same as with the previous image. 

Figure 12: Both SPOT images in Point Measurement viewer

A total of 11 points were collected.

Section 5: Automatic tie point collection, triangulation and ortho resample

The Automatic Tie Point Generation Properties icon was selected from the Point Measurement tool. The specifications were set to the following:

General Tab:

Image used: All Available

Initial Type: Exterior/Header/GCP

Image Layer Used: 1

Distribution Tab:

Intended Number of Points/Image: 40

Keep All Points: unchecked

The model was run and tie point accuracy was checked. The Point Measurements were then saved and closed.

The Block Triangulation Properties window was opened. The specifications were set to the following:

General Tab:

Iterations With Relaxation: 3

Image Coordinate Units for Report: Pixels

Point Tab:

Ground Point Type and Standard Deviations: Same Weighted Values

X, Y, and Z: 15

Advanced Options Tab:

Simple Gross Error Check Using: checked

Times of Unit Weight: 3

The triangulation was run and the Triangulation Summary Dialogue was saved.

Start Ortho Resampling Process was clicked to begin orthorectification. The specifications were set to the following:

General Tab:

DTM source: DEM

Output Cell Sizes: 10

Advanced Tab:

Resampling Method: Bilinear Interpolation

Add Single Output was selected and the second SPOT image was set as the input image. Current cell sizes were used. The ortho resampling process was run and completed.

Results

Part 1: Scales, Measurements, and Relief Displacements

Section 1: Calculating scale of nearly vertical aerial photographs

It is relatively rare to have a ground distance measurement to determine scale with. But when available, the photo distance is divided by the ground distance to obtain the scale. The scale of the image for this section was S = 1 : 40,331”.

If a ground distance is not known, scale can also be calculated from flying altitude above sea level, elevation of land above sea level, and focal lens length. The scale from the image was S = 1 : 38,509 mm.

Section 2: Measurement of areas on aerial photographs

The polygon in figure 3 was drawn and the perimeter and area of it were automatically computed. They were (in different units, with could be chosen from a dropdown list):

Area: 38.10 hectares, 94.14 acres

Perimeter: 4116.38 meters, 2.56 miles

Section 3: Calculating relief displacement from object height

The calculation of relief displacement is used to correct those distortions. Using the final RD of 4.21”, it can be determined that it will need to be a negative correction to account for the positive RD.

Part 2: Stereoscopy

Section 1: Creation of anaglyph image with the use of a digital elevation model (DEM)

This type of anaglyph image requires aerial imagery and a DEM. The resulting anaglyph below can be viewed using polaroid glasses. 

Figure 13: Anaglyph using DEM

Section 2: Creation of anaglyph image with the use of a LiDAR derived surface model (DSM)

This type of anaglyph image requires aerial imagery and a LiDAR surface model. The resulting anaglyph below can be viewed using polaroid glasses.

Figure 14: Anaglyph using LiDAR surface model

Part 3: Orthorectification

The orthorectification process for two images taken by a spot satellite was very successful. Below is an image of both images together. Features are aligned well, which can be seen especially well using the swipe tool.

Figure 15: Result of orthorectification of two previously distorted images

Sources

Wilson, C. (2019). Photogrammetry. Geog 338: Remote Sensing of the Environment Lab 7.