ES 555 Lab Exercise on
Digital Orthophotos

James S. Aber


Digital orthophotos are derived from original aerial photographs that have been scanned and georectified. These images have been resampled to fit accurately onto a ground coordinate grid with very high pixel resolution--usually 1 or 2 meters. The resulting raster images are refered to as digital orthophoto quadrangles (DOQ). DOQs cover a ground area 7˝ x 7˝; minutes of latitude and longitude in extent. DOQ quarter quadrangles (DOQQ) have 1-meter resolution and encompass 3¾ x 3¾ minutes of ground area. Positions of DOQs are keyed to USGS 1:24,000-scale topographic map sheets. The original input airphotos come from the National Aerial Photography Program (NAPP) or the National High Altitude Aerial Photography Program (NHAP).

See DOQ description.
See NAPP description.
See NHAP description.

Digital resampling and georectification remove the geometric distortions that are inherent in the original air photos. This means that DOQs may be utilized for accurate measurements of distance and direction--just like a map. DOQs may be used as one layer in a GIS database. For example, DOQs are often employed as the "background" layer, on which other geographic data may be overlaid for display or analysis purposes. The digital character of DOQs also allows for various kinds of image enhancement techniques to improve the visual display or to highlight specific features.


The following datasets are small subscenes extracted from the DOQQ for Marion, Kansas--scene ID 1241_ne. The DOQQ is panchromatic (b/w visible) and has 1-meter resolution. The datasets are provided as "raw values" that have not been digitally enhanced for display purposes. Transfer the following datasets via FTP into your personal computer work space. Note: you need both *.rdc and *.rst files for Idrisi Selva.

File Name Feature
Business district & park
Cemetery & athletic field
Golf course

Marion County courthouse, a stately turn-of-the-century, limestone building in the central business district of the City of Marion. Photo © J.S. Aber.
A well-preserved limestone building on Marion's main street (Kansas highway 256), now home to the First Federal Savings Bank. Photo © J.S. Aber.
Old cut-limestone building in downtown Marion, now home to Gambino's Pizza. Photo © J.S. Aber.

1. Check Metadata to examine the documentation file for MN_1. How many rows and columns does this image have? What is the ground area (km²) covered by the image?

Now call on the Display button (map icon) to show the image on screen, as follows. Select MN_1, choose the "Grey scale" palette; click off "Title" and "Legend." Finally click OK, wait a moment for image to appear, then hit the "End" key to enlarge the image. Default autoscaling is a quick-look means to create a display.

2. Describe the appearance of the following features, paying attention to tone, size, shape, pattern, shadows, and other visual attributes. You may use the "zoom" and "pan" functions to enlarge selected features.

3. Return the image to the full scene, and enlarged it to fill the working window. Move the cursor around, and carefully note row and column numbers. Are all rows and columns displayed in the image? Why do you suppose this is the case?

The results of autoscaling represent a minimal level of display enhancement. To improve the appearance of this image, you will now utilize different stretch operations. Read about the STRETCH module in the Help section. Now run STRETCH (double arrow icon) as follows. Select MN_1 as the input image, and name MN_1S as the output image. Click on "Linear with saturation" and enter 5.0 as the percent to be saturated. Leave other selections in the default modes. Under "Output documentation" enter an appropriate title, and enter "linear stretch 5%" as value units. Finally click OK (twice) and wait a few seconds for image to appear. You may wish to redisplay without autoscale, title, and legend.

4. Describe the change in appearance for the linear stretch image compared to the autoscaled image.

Next repeat the procedure for STRETCH, only this time choose the "Histogram equalization" option. Name the output image MN_1H, provide an appropriate title, and enter "histo stretch" for value units. Display the histogram stretch and compare it to the linear stretch image.

5. Describe the different appearance of the histogram stretch image compared to the linear stretch image.

6. What numerical techniques are used to create the linear stretch and histogram stretch images?

7. Of the various images you have created, which one do you prefer? Explain your answer.

Now turn your attention to MN_2 and MN_3, which have the same geometrical characteristics as MN_1. Make your favorite image to display each one, and answer the following questions about features visible in each of the three images.

8. Questions for MN_1, the central business district.
9. Questions for MN_2, the cemetery and athletic field.
10. Questions for MN_3, the golf course.
11. Based on these images, give your rating of interpretability using the civilian NIIRS criteria.

As your final activity, use the Map Composer to create a complete map composition. Pick your favorite image, and add the following elements--appropriate title, subtitle (your name/date) and scale bar (north arrow is optional). Name your map composition MARION and save a digital image to turn in (submit jpg file via e-mail).

Turn in

Return to SFAP schedule.
ES 555 © J.S. Aber (2014).