Spatial Analysis Based on Raster Data Processing - Introducing Geographic Information Systems with ArcGIS - page 490

Identify tool to examine four values (to three significant digits). _______, _______, _______,
_______. Use the CellTool to light up the whole raster with values.

21. Ran_Ras contains numbers in the range zero to one. Suppose that instead you wanted to have
integer numbers between 1 and 10 put in the cells. You could use the Int function (make into
an integer) in the Raster Calculator on Ran_Ras. You will find Int in the box to the right of the
Boolean buttons. Place this expression in the Raster Calculator:

Int((“Ran_Ras” * 10) + 1)

Put the output, Ran_Ras_1_to_10, in Ras_Expr.gdb. This produces an Integer raster, so it
has an attribute table. How many values of “1” did you get? ________. How many of “7”?
________. Based on the digits from 1 to 10 being evenly distributed over 42 cells, about how
many of each digit would you expect in the raster? _______. Check it out with the attribute
table.

A second floating-point raster demonstration: The cosine, say, x , of an angle between zero and a right
angle is a number between 1 and 0. The arc cosine function finds the angle whose cosine is x . So we
could use our Ran_Ras raster (which you recall is bounded by zero and one) to make a raster of random
angles, using the Acos function.

22. Start the Raster Calculator. Find the Acos Trigonometric function in the list of the Raster
Calculator.

23. Create the expression

Acos(“Ran_Ras”)

Name the output raster Random_Angles and put it . . . you know where by this time. The
floating-point raster is added to the map. Check out the results. You may have expected to see
angles between 0 and 90. But this function, which you will discover if you check the Help file,
produces resulting angles expressed in radians, not degrees. There are 2*pi radians in the 360
degrees of a full circle. So one radian is about 57.296 degrees. Thus you get values bounded
by about zero and pi/2 (about 1.57).

24. To convert the Angles raster to degrees (let's say integer degrees), use the Raster Calculator
one more time:

Int(“Random_Angles” * 57.296)

Make the new raster called Random_Angles_Degrees. Now check out the values. You are
back to an Integer raster, so you have an attribute table. What's the smallest? ________
The largest? ________

You now have a grasp (or at least an inkling) of how both integer rasters and floating-point rasters
operate. You could play indefinitely with the immense number of functions and rasters. Better that you
go on to Project 8-2, where you solve the Wildcat Boat problem with rasters.