Stereoscopy, Orthorectification, & Calculations

INTRODUCTION

The following lab demonstrates knowledge in stereoscopy and performing orthorectification. Key photogrammetric tasks were performed on aerial photographs and satellite imagery to understand calculations of photographic scales, measurements of areas and perimeters, and calculating relief displacement. First, measurements were made from aerial imagery to calculate scales, areas, and relief displacement. Next, anaglyph imagery was created using a digital elevation model (DEM) and a digital surface model (DSM). Finally, orthorectification was performed to produce a planimetrically true orthoimage using ground control points (GCPs).

METHODOLOGY

Calculations from Aerial Imagery

Scales

To calculate scale from a nearly vertical image, focal length is divided by the flying height of the aircraft. Flying height is calculated from the altitude above sea level (ASL) of the exposure station minus the elevation of terrain.

Areas

To calculate measurement of area on aerial imagery, the area is digitized using a measurement tool which gives you an output of desired units. 

Relief Displacement

To calculate relief displacement, the height of a tall object is measured and converted into a distance using the aerial image's scale. This value is then multiplied by the distance from that object to the principal point of the object. This product is then divided by the height of the camera above the local datum of that image. 

Stereoscopy

To correct for relief displacement and better view height and changes in elevation, anaglyph imagery is created for a 3D effect. Original imagery of Eau Claire with relief displacement was combined with a digital elevation model at a 10-meter spatial resolution. Next, the same function was performed with a LiDar-collected, 2-meter spatial resolution digital surface model. 

Orthorectification

To correct for spatial anomalies, orthorectification was performed on two aerial photos to produce a seamless transition between the two. Both images were uploaded to IMAGINE Photogrammetry Project Manager. The point measurement tool was then activated to collect ground control points between each image and a separate reference image without spatial anomalies. When matching GCPs, the coordinates between images were within 10 meters of each other to maintain accurate corrections. After matching three GCPs, the automatic drive was selected allowing the program to estimate matching GCPs on the reference image.

After 12 GCPs were collected for both images based on the reference image, automatic tie point collection was performed to match GCPs collected from both input images. Triangulation was then performed to match the tie points collected in the overlapped area of the two images. Finally, ortho resampling was performed to correct the original spatial anomalies of both images. 

RESULTS

Calculations from Aerial Imagery

Scales

If the distance between two points were measured by an engineer's chain to be 8,822.47 ft and according to aerial imagery, the distance measured 7 inches, the scale of the image would equal 1:38,498.

If an aircraft took aerial photography at 20,000 ft. ASL of Eau Claire with a 796 ft. ASL elevation, with a focal lenth of 152mm., the scale of the photograph would equal 1:38,509.

Areas

Imagery from ERDAS was carefully digitized and automatically given an outputted area measurement according to the polygon drawn. 

Relief Displacement

The height of the smokestack in the image measured 0.5 in., which coverts to 133.71 ft., using a scale of 1:3,209. The 133-foot smokestack was then multiplied by the distance to the principal point of the image (10.5) and divided by the height of the camera above the local datum of the image (3, 980). As a result, the calculated relief displacement of the image is 0.35 away. 

Stereoscopy

The original Eau Claire aerial image shows clear relief displacement that shows distortion within the image. This can be observed in Figure 1 by the tall buildings of Towers North, a tall campus building located in Eau Claire.

A stereoscopic view of Eau Claire was then produced using a 10-meter spatial resolution digital elevation model which gives an enhanced view of depth and elevation using three dimensions. This is illustrated in Figure 2 and 3. However, polaroid glasses are needed to view the imagery in 3 dimensions.

Figure 2: Input photos of relief displacement and 10-meter DSM

Figure 3: Output stereoscopic image from Figure 2 input images

The same process was repeated, but instead a LiDar-collected DSM at 2-meter spatial resolution was used for the anaglyph image. The results showed a much more in-depth representation of elevation change and height of surface features. 

Figure 4: Enhanced anaglyph photo using LiDar-collected DSM

Orthorectification

While collecting matching GCPs between both input images and the spatially-accurate reference image, all coordinates were within 10 meters to preserve an accurate orthorectification as shown by Figure 5.

Figure 5: Collecting GCPs from input image to a spatially-correct reference image

After collecting all GCPs for both input images, a summary is shown in Figure 6 of the automated tie points generated in the Photogrammetry Project Manager.

Figure 6: Automatic tie point collection between both images and the reference image

The resulting tie points for each input image is shown in totality by Figure 7.

Figure 7: All tie points gathered between the two input images

Figure 8 then shows an overview of both images after performing triangulation calculated based on the tie points generated.

Figure 8: Triangulation from tie point collection

Finally, Figure 9 shows the orthorectified image after being resampled. Both inputted images now have all spatial anomalies orthorectified to produce a seamless transition and accurate spatial data.

Figure 9: Spatially-accurate orthorectified output imagery

SOURCES

National Agriculture Imagery Promgram (NAIP) images are from United States Department of Agriculture, 2005.
Digital Elevation Model (DEM) for Eau Claire, WI is from United States Department of Agriculture Natural Resources Conservation Service, 2010.
Lidar-derived surface model (DSM) for sections of Eau Claire and Cheippewa are from Eau Claire County and Chippewa County governments respectively.
Spot satellite images are from Erdas Imagine, 2009.
Digital elevation model (DEM) for Palm Spring, CA is from Erdas Imagine, 2009.
National Aerial Photography Program (NAPP) 2 meter images are from Erdas Imagine, 2009.