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

By James H. Nicholson and Ellen Vaughan

Charleston County School District CAN DO Project for the EarthKAM Teacher Training Institute

Introduction

EarthKAM teachers have often expressed a desire to be able to use a single software program to perform image processing, image annotation and quantitative measurements. Most often they have indicated a preference for Adobe PhotoShop. PhotoShop, however, did not support scaled measurements. NIH image, which could be used for measuring, had proven difficult for many to use. This article will give instructions for using PhotoShop v5.0 for quantitative linear and area measurements.

Click on pictures for a larger version

STEP 1. SCALING THE IMAGE FOR MEASUREMENT

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While PhotoShop does not provide a way to directly scale an image in kilometers, it does have the ability to accurately measure image structure in photographic and imaging units. Therefore we need to establish a constant relationship between kilometers and image pixels (individual image units). Fortunately, because most images for measurement do not to be very high resolution, the simplest one to one (1 pixel = 1 kilometer) ratio will often be quite suitable. For high resolution measurements, or measurements of a smaller cropped image, a 10 to one (1 pixel = .1 kilometer) scale will be more accurate and almost as simple to use.

To set the PhotoShop system to use pixels as the default unit:

  1. Choose File > Preferences > Units & Rulers
  2. Set Units = Pixels

To scale the image dimensions

  1. Choose Image > Image Size.
  2. Make sure that Resample Image is selected. The interpolation method should be left at Bicubic (default).
  3. To maintain the current proportions of pixel width to pixel height (shape), select Constrain Proportions. This option automatically updates the width as you change the height, and vice versa.
  4. Under Pixel Dimensions, enter values for Width and Height based on the frame height and frame width size listed on the EarthKAM Detailed Data Sheet for the image. For this example frame height = 142.08 (km) and frame width = 212.58 (km). You would enter 142 in the height box and the width will update automatically.
  5. The new file size for the image appears at the top of the Image Size dialog box with the old file size in parentheses.
  6. Click OK to change the pixel dimensions and resample the image.

In the new smaller image you have now created, every pixel represents 1 km in linear measurements and1 sq. km in area measurements. Save the image under a new file name indicating that this is a special scaled image for measurements. (STS08901102125_scale.jpg)

 

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Introduction

Part 2 - Linear Measurements

Part 3 - Area Measurements

Adobe Acrobat Version (PDF) (500K)

Microsoft  WORD 97 Version (DOC) (1.3M)