Photogrammetry

Introduction

Photogrammetry is study of the geometric characteristics of aerial photographs. In order to extract correct and accurate geospatial information from imagery, the images need to be free of errors and spatially accurate. Specifically, photos need to be free from three major errors: Perspective Convergence, Scale Variation, and Relief Displacement.

Perspective Convergence is an error that occurs when pixels are not the same size, this error stems from camera tilt, where the camera captures an image at an angle. The angle of the captured photos makes pixels further away from the camera squeezed. 

Figure 1. Respective Convergence.  

Scale Variation results from differences in terrain, resulting in various scales for the different elevations of terrain captured in the aerial imagery. 

Figure 2. Scale Variation. 

Relief Displacement is an error that occurs when tall objects appear to be leaning towards or away from the principle point of the photograph. If the object is taller than the datum the object will appear to lean away from the principle point. If the object is lower than the datum the object will appear to lean towards the principle point. The taller or lower the object is from the datum the greater the displacement. If the object is further away from the principle point, the object will also appear to have greater displacement. 

Figure 3. Relief Displacement. As objects vary in distance from the principle point, the relief Displacement will be increased. In the image to the right, the points have been corrected of the relief displacement error.

Figure 4. Relief Displacement In An Aerial Photograph. Relief Displacement as depicted on the upper campus of UWEC. The tower is leaning away from the Principle Point and will have to be corrected.   

Once the images have had the errors removed, the images are plotted as an anaglyph, a type of 3 dimensional image, and used for Digital Elevation Models or Digital Terrain Models.

The process of Orthorectification is the simultaneous removal of positional and elevation errors from one or more satellite images. The process also obtains real world X, Y and Z coordinates of pixels on aerial photographs.

Methods

We wanted to compare two anaglyphs using two forms of ground control points. One of the ground points will be from a Digital Elevation Model (DEM) at 10 meter spatial resolution, while the other will be from a LiDAR derived Digital Surface Model (DSM) at 2 meter spatial resolution. 

For the first Anaglyph a photo of Eau Claire was combined with DEM shown below (Figure 5).  The second Anaglyph was also a photo of Eau Claire but this time it was combined with the LiDAR-Derived DSM (Figure 6). As noted above, not only with the Anaglyphs be different because they are based on different ground control points in the form of a DEM and DSM, but the spatial resolution of the output images will also be different. 

Figure 5. Anaglyph 1: Derived from DEM.

Figure 6. Anaglyph 2. Lidar-Derived DSM.

For Orthorectification, we will be using Erdas Imagine Lecia Photogrammetric Suite to accomplish the creation of a planimetrically true orthoimage.

We will be using two corrected orthoimages as a source of ground control images of panchromatic imagery at the 10 meter spatial resolution. The images displayed below are the images that will be corrected, both are SPOT panchromatic images. When displayed in one viewer, both panchromatic images share the same space, one overlapping the other. 

Figure 7. Original Images Before Orthorectification.

After putting in one of the panchromatic images and a control image, ground control points are placed throughout the panchromatic images, using the known corrected orthoimages. 

Figure 8. Ground Control Points.

After placing all of the control points, the second panchromatic image can now be added to the block model. After adding ground control points to the second pan chromatic image, the automatic tie point collection can occur, followed by triangulation of the images. The output images then need to be resampled and finally output using the orthoresampling process, which generates an output image of both of the panchromatic images with spatial accurate X,Y and Z coordinates. 

Results

Both Anaglyphs are displayed below. The anaglyph on the left is Anaglyph 1, which was created using the DEM. You will only be able to see the 3D effect if you have polarized glasses ever, the effect of the 3D representation is stronger in Anaglyph 2. Anaglyph 2 contains imagery and a DSM of higher spatial resolution source imagery, as well as not containing relief displacement in the original images. If you have polarized glasses, you will be able to detect the differences in the 3 dimensional effect of the images. 

Figure 9. Anaglyph 1 and Anaglyph 2. Anaglyph 1 uses an Eau Claire image at 1 meter spatial resolution and a DEM to create the anaglyph (left). Anaglyph 2 is a image of Eau Claire at 2 meter spatial resolution and a LiDAR-derived DSM (right).

After completing the model for Orthorectification, both panchromatic images are now overlaid into one image (Figure 10). The geographic features in the photo below are lined up.

Figure 10. Orthorectified SPOT Panchromatic Images. After the tool has been run we can see that both of the Spot Panchromatic Imagery are now in the correct position and orthorectified and are now overlapping. From a distance we can see the boarders of the images. 

A close up of the two orthorectified panchromatic images (Figure 11), shows a seamless transition between the two images. 

Figure 11. Closeup of the Orthorectified SPOT Panchromatic Images. When zoomed in on the overlap of the two orthorectified images we can see the high degree of  spatial accuracy at the boundaries, where a seamless transition between features exists. 

Sources

Chippewa County Governemnt | LiDAR-derived Surface Model of Chippewa County , WI (DSM). 

Eau Claire County Government | LiDAR-derived Surface Model of Eau Claire, WI (DSM). 

ERDAS Imagine | SPOT Satellite Images ( Panchromatic Images A and B). 2009.

ERDAS Imagine | Digital Elevation Model (DEM), (Pam Springs, CA DEM). 2009 

ERDAS Imagine | National Aerial Photography Program. (NAPP Images at 2 meter resolution). 2009.

Geog 258: Maps and GIS. http://gis.depaul.edu/shwang/teaching/geog258/lec7.htm Retrieved December 7, 2016.

United States Department of Agriculture | National Agriculture Imagery Program (NAIP). 2005.

United States Department of Agriculture | USDA Natural Resources Conservation Service. (Eau Claire Digital Elevation Model (DEM)). 2010.