ES 555 Image Processing II
James S. Aber
The second exercise is based on a digital kite aerial photograph (KAP) from the Maple Wood Cemetery in Emporia, Kansas. This portion of the cemetery is the older part located along Lincoln Street between 15th and 20th, which includes Civil War burials and a potter's field.
The exercise image is a near-vertical view taken with a Canon PowerShot 230 (Digital Elph) on 31 March, 2003. Transfer the following file via FTP into your personal computer work space: emp_cem1.jpg.
||Overviews of the Maple Wood Cemetery. Left: looking toward the southeast with downtown Emporia in the background. Right: view eastward including the potter's field section (lower left).
This picture depicts an early spring scene, in which cool-season grass is just beginning to green up. The study site is the potter's field, in which many unmarked graves date from the period 1880s to 1920s. Some of these graves appear as darker green rectangles in the image. Various geophysical methods were tested for identifying disturbed soil of burials at this site. Prior to KAP, a large north arrow was positioned on the ground along with survey markers.
||Raw image of the Emporia cemetery study site. Notice the north arrow (4 m long) and survey markers (1 m˛). North is toward right. Click on small image to see larger version.|
Your goals are to orient the north arrow straight up on the image and to determine the image resolution. Also you will carry out processing steps learned in exercise I. Begin by noting image size (height and width). As a first step, rotate the image 90° counterclockwise. This brings the north arrow into approximate upright position.
The suburban housing area (top of image) is of no interest. You may crop off most of the house, but retain the treeline, as it marks the northern boundary of the cemetery property. Your resulting image should be roughly square in shape.
||Rotated and cropped image of the Emporia cemetery study site. Click on small image to see larger version.|
Now rotate the image again to align the north arrow straight up. Use the "Rotate Canvas: Arbitrary" as you did in exercise I. This step may require several test rotations to achieve the best alignment. Use the zoom function (Ctrl +/-) to examine the arrow closely.
The arbitrary rotation created some blank borders around the image. Crop these blank borders off, retaining as much of the study site as possible.
- 1. What rotation achieved a best position for the arrow? Indicate direction (CW or CCW) and angle in degrees.
||North arrow aligned straight up and image cropped to study site. Click on small image to see larger version.|
Now carry out additional image enchancement, including brightening and sharpening, to improve the visual display of this scene. Do not annotate the image yet.
At this point, you are ready to determine the image resolution. This is the linear ground distance, also called ground sample distance (GSD), represented by each pixel in the image. Enlarge the arrow until you can see clearly individual pixels (Ctrl +).
Highly enlarged view of the north arrow.
Each pink-and-white square in the arrow is 1 m˛, and total length of the arrow is 4 meters. Count the number of pixels (lengthwise) for the three squares. Divide this result by 3 to get pixels per meter. Use the following formula to determine pixel resolution.
100 ÷ pixels per meter = resolution (in centimeters)
For example: 100 ÷ 25 pixels per meter = 4 cm
Next you will import the image into Idrisi. Using Adobe Photoshop, convert the jpg image file to bmp format; use the "save as" function and select bmp as the file type.
- 2. What is the resolution you calculated for this image? Give your answer in centimeters, meters, and inches.
- 3. What are the approximate dimensions of the whole scene? Give your answer in meters. Hint: multiply rows and columns by resolution.
- 4. Based on the resolution of this image, what are the smallest features that you can positively identify? Be sure to enlarge image to 100% display size so that details are visible. Explain how you recognized these objects.
Now import the bmp file into Idrisi: File/Import/Desktop Publishing Formats/BMPIDRIS. The cemetery image should display automatically with a default title and 24-bit composite color palette. Now update the metadata with appropriate spatial information (leave other items in default values).
- 5. Compare the size (in bytes) of the jpg file to the bmp file. What happened to file size?
Save the new metadata (icon in lower left corner of Idrisi Explorer). Note: the new metadata will not take effect on the displayed image. Close, then reopen the Emporia cemetery image for the new metadata to apply—you should see the new title.
- Maximum x (= columns x resolution, in meters).
- Maximum y (= rows x resolution, in meters).
- Resolution (in meters).
Proper spatial metadata are necessary for quantitative analysis of the scene. For example, determine the area depicted in this image. Multiply max x times max y.
Create a map composition for the cemetery image that includes a title, subtitle (your name), and scale bar with appropriate units. Save your final composition in bmp format with the name cemetery. Finally convert the bmp file into jpg format for submission.
- 6. What is the area in square meters?
Note: your image now has a scale bar based on the north arrow near scene center. However, many subtle distortions are present in the image geometry, so the scale bar is not absolute for all portions of the image (see lecture on photogrammetry).
- Written answers (1-6).
- cemetery.jpg image file.
Return to SFAP schedule.
ES 555 © J.S. Aber (2014).