image32.jpg (15454 bytes) USING ADOBE PhotoShop TO MEASURE EarthKAM IMAGES

PART 3 - Grids & Area Measurements

Placing a Measurement Grid Over the Image

Grids appear as lines that float over the entire image and do not print. You can move, remove, or lock a grid to avoid accidentally moving it. The grid appears by default as nonprinting lines but can also be displayed as dots. The grid is useful for quick size estimates.

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1 Choose File > Preferences > Guides & Grid.

2 For Color, choose a color that will stand out from the image.

3 For Style, choose a display option for grid (solid or dashed line).

4 For Gridline Every, enter a value of 10 to display a major gridline every 10 km.

For Subdivisions, enter a value of 2 to subdivide the grid into 5-km blocks.

5 Click OK.

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To Make Manual Area Measurements

 
  1. To make an interactive (operator controlled) area measurement, select the lasso tool. Click on the lasso icon (2nd from the top left) on the PhotoShop Tool Bar.
  2. Hold down your mouse button and draw around the area you wish to measure. The measurement will close in a straight line to your starting point when you release the mouse button.
  3. Choose Image > Histogram
  4. In the Histogram dialogue box in the lower left-hand corner is a value for Pixels. This value is the total number of square kilometers in the area you have drawn.
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Lasso options - feather size should be set to zero

To Make Multiple Area Manual Measurements

To measure multiple area measurements (series of lakes) make the first manual measurement as described above. Hold down the Shift key and repeat the process for the additional areas of interest.

 

To Make Automatic Area Measurements

 
  1. To make an automatic (computer controlled) area measurement, select the magic wand Tool. Click on the magic wand icon (2nd from the top right) on the PhotoShop Tool Bar.
  2. Click on the area of interest (distinguished by color and/or brightness). If the area selected is incorrect, double click on the magic wand and change the Tolerance value in the option box. A larger number will select a larger area and a smaller number will select a smaller area. If no degree of tolerance gives a satisfactory automatic measurement, you should do a manual measurement instead.
  3. Choose Image > Histogram
  4. In the Histogram dialogue box in the lower left-hand corner is a value for Pixels. This value is the total number of square kilometers in the area you have drawn.

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Making Multiple Area Automatic Measurements

To make an automatic measurement of multiple areas that share similar color and density, proceed as described above for a typical area. Choose Select > Similar. If the areas are similar enough they will be selected together. If the selection is too small try Select > Grow. If it is not possible to get a satisfactory automatic multiple selection, it is probably because the subject areas are not distinctive enough. Use the method for measuring multiple area manually.

 

Introduction

Part 2 - Linear Measurements

Part 3 - Area Measurements

 

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