ES 555 Lab Exercise on
Digital Orthophotos

MARION, KANSAS
James S. Aber

Introduction

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 and NHAP descriptions.
See DOQ standards.

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.

Exercise

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 Andes.

File Name Feature
MN_1.*
Business district & park
MN_2.*
Cemetery & athletic field
MN_3.*
Golf course

Marion County courthouse, a stately turn-of-the-century, limestone building in the central business district of the City of Marion. Photo date 4/91; © 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 date 4/91; © J.S. Aber.
Old cut-limestone building in downtown Marion, now home to Gambino's Pizza. Photo date 4/91; © J.S. Aber.

1. Use the Metadata button (4th icon from left) 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 (bottom of Composer box) to enlarge selected features.

  • "Main" street (Kansas highway 256), running E-W across scene.
  • County court house, right center of scene.
  • Park area along east side of scene.
  • Farm implement dealer, left edge of scene.
  • Grain elevator, lower right portion of scene.

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 Idrisi Andes help section, under program modules--display. 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.

  • The grain elevator was built next to a railroad, which is the diagonal NW-SE linear feature. What is the current status of this railroad?
  • What kind of vehicle parking pattern is evident on "main" street (Kansas highway 256)?
  • What time of day was the original airphoto taken? What season of the year was the original airphoto taken? Explain your reasoning.
  • Does flooding appear to be a possibility for any portion of this scene? Explain your answer.

9. Questions for MN_2, the cemetery and athletic field.

  • What is the diameter of the "circle" drive in the cemetery? Hint: check grid coordinants as you move mouse pointer across the image.
  • Are you able to identify individual tombstones in the cemetery? Explain your answer.
  • The athletic area is designed for multiple uses. What kinds of fields are present; what sports can be played here?
  • Just south of the athletic field, there is a bright white, round feature. What is it? How are you able to identify this object?

10. Questions for MN_3, the golf course.

  • Is this a 9-hole or 18-hole golf course? How were you able to make this determination?
  • What total area (in km˛) is occupied by the golf course and club-house complex?
  • Does this appear to be a technically difficult golf course for players? Explain your answer.
  • Speculate on why most of the golf course appears light in tone compared to surrounding agricultural fields.

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), scale bar, and north arrow. 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 (2008).