GEOG 380

Map, Air Photo, and Satellite Interpretation

Spring, 1999

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LAB 8:


(Due Thursday, April 29)


Land Cover Interpretation

This lab consists of interpreting land cover from an aerial photo. A standard classification for land cover was produced by the USGS in 1976. Chapter 8, pages 206-222 in Avery and Berlin, discusses land cover mapping and the USGS system. If you haven't yet done so, look over pp. 215-221, especially Table 8-1, page 207, and Figure 8-20, page 221. In this lab, you will also find the area occupied by each cover type, by means of a dot-planimeter which you will make. Each dot on the sampling grid represents a given area, so by multiplying the number of dots by the area per dot, you can find the total area. (Note: This counting assumes no distortions in the photo, especially those due to elevation differences on the ground. Hence your count is likely to be inaccurate to some degree.)


For this lab you will need:

*an aerial photo pair of somewhere in Sonoma County (note the area indicated when you pick up the photos)

*pocket stereoscope

*clear plastic overlay

*some drafting tape

*grease pencil

*dot-grid overlay

*ink pen for dot-grid overlay


A. Marking Cover Types on the Overlay

Examine the photo for a few minutes. Try to get a sense of the kinds of land cover that are present, and the criteria you are using to judge identifications, such as tone, texture, and shape. Tape the plastic overlay onto the photo along one edge, so that you can lift the overlay and look at the photo as needed. Then interpret land-cover types on the photo, and use the grease pencil to mark the perimeter of each cover type. Stray marks and errors can be erased with a paper towel or tissue. Follow the guidelines found in Avery and Berlin, especially these pointers:

*Make all your designations as polygons, not as lines or points. For example, don't bother drawing in stream channels as lines -- show only the riparian area if extensive enough, e.g., as category 51 or 62.

* Minimum parcel area on the map should be about 3-4 mm on a side. (see Figure 8-20)

*Try to interpret land-cover classes down to at least Level II (e.g., 21 for cropland, 53 for reservoirs). You can use your own Level III types as you see fit. (see Table 8-4)


B. Make a Dot Planimeter

You will need to know the scale of your photo in order to make a dot planimeter. So first calculate the scale (RF) of your air photo by comparing it with a 7.5' topographic map of the area, as you did in an earlier exercise. Using the metric scale on a ruler, and the Representative Fraction for your photo, draw a grid on a piece of paper with squares that represent 100 meter squares on the ground. For example, if your air photo has a scale of 1:24,000 (it's probably larger scale than that), 1 cm on the map equals 240 m on the ground. But you want to know what 100 m will be, so divide 100 by 240. The result tells you that a bit over 4 mm (~4.2 mm) equals 100 m on the ground. This is sufficiently accurate for our purposes. (NOTE: THIS ISN'T THE CORRECT SCALE FOR YOUR PHOTO, SO YOU MUST CALCULATE THIS YOURSELF.) After you make your grid of about ten rows on each side (100 squares), lay the acetate over the grid and tape it down. Place a dot of ink in the center of each square. The dots are 100 meters apart. Each square represents a hectare.


C. Estimating Area of Each Cover Type

Your next task is to determine the area covered by each major land-cover type on the photo. Use your dot planimeter to count the area of each cover type. Remove the overlay from the photo, place it on a light surface (e.g., a white sheet of paper), and place the dot grid over the land-cover overlay.

Tally up the number of dots per cover type, using the accompanying table. Count each dot based on the cover type directly beneath the dot. If a dot falls on a line, let us count it as the cover type to the north of the line (there are other possible rules, but this one will do for us). Because your dot planimeter doesn't cover the entire photo, count all the dots for each type separately. You may have to move the dot planimeter to cover large areas, in which case mark your ending boundary with the grease pencil, then erase the mark afterward.

The grid is constructed so that there is one hectare per dot. Therefore your estimate of coverage of each cover type is the same as the number of dots. For many dot grids, you would need to convert number of dots to ground area.

Finally, calculate the percentage area in each cover type on your photo by dividing by the total area and multiplying by 100.

(Level I)
Number of Dots
Percentage of
Total Area
Urban/Built Up









Forest Land









Barren Land







When finished, turn in your completed overlay along with the air photo and the lab exercise, and return other materials.

Go to Lab 9

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Page last edited 4/18/99