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.
STEP 1. SCALING THE IMAGE FOR MEASUREMENT
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)
Using rulers
When visible, rulers appear along the top and left side of the active window.
Markers in the ruler display the pointer’s position when you move it. The numbers
on the ruler now reflects km in the image.
To display or hide rulers:
Choose View > Show Rulers or Hide Rulers.
To Make Linear (point to point) Measurements
Select the measure tool
The measure tool has a minature ruler icon located as the eight tool down on the
left hand side of the standard Adobe PhotoShop Tool Bar. The measure tool
calculates the distance between any two points in the work area. When you
measure from one point to another, a non-printing line is drawn and the Info
palette shows the following information:
• The starting location (X and Y).
• The horizontal (W) and vertical (H) distances traveled from the x and y axes.
• The total distance traveled (D).
• The angle measured relative to the axis (A).
• Optionally set a second measuring line to create a protractor. This lets you
measure angles separated from the x and y axes, and view two distances
traveled (D1 and D2). Click on the line start-point while holding down the ALT
key. The angle between the two base
lines is measured. This method could be
used to measure a direction compared to
the Look Direction angle from the
detailed data page.
All measurements except the angle are
calculated in the unit of measure
currently set in the Units & Rulers
preference dialog box (In this case pixels
but remember that 1 pixel = 1 km.
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.
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.
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.
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.
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.