Structures for Storing Geographic Data - Introducing Geographic Information Systems with ArcGIS - page 275

The errors that can occur when using floating-point numbers with decimal parts are illustrated by the
following steps.
5. In ___IGIS-Arc_ YourInitials \Other_Data
there is a text file that looks like this:
Polyline
5 0
0 1000.00 4000.00 0.0 0.0
1 1002.10 4000.00 0.0 0.0
2 1002.10 4002.10 0.0 0.0
3 1000.00 4002.10 0.0 0.0
4 1000.00 4000.00 0.0 0.0
6 0
0 1000.00 4000.00 0.0 0.0
1 1002.10 4002.10 0.0 0.0
7 0
0 1002.10 4000.00 0.0 0.0
1 1000.00 4002.10 0.0 0.0
END
Verify that the text file describes a square whose sides are 2.1 units and which contains
two diagonals, which cut the square into four triangles. Start ArcMap with a blank map. From
ArcToolbox start the tool “Create Features From Text File” in the toolbox 11 Samples > Data
Management > Features. For the input Text File use the file above from Other Data. The decimal
separator is a period. Put the output feature class in
___IGIS-Arc_ YourInitials \TextToFeature
\TextToFeature_DB.mdb\TextToFeature_DS
calling it Four_Triangles_Lines. Make a polygon feature class called Four_Triangles_Polygon by
running the Feature to Polygon tool
in Data Management Tools > Features. Input features should be the Four_Triangles_Lines
feature class. Make the Output Feature Class Four_Triangles_Polygon in the same location
as Four_Triangles_Lines. Click OK. Add both feature classes to the map. Look at the lines
feature class attribute table. You can easily calculate that the perimeter of the square as
8.4 (4 x 2.1). What does the lines table show? _______________________. Look at the polygon
feature class attribute table. The area of each triangle is 1.1025. What does the polygon table
show? _______________________.
The Shape_Length of each line and the Shape_Area of each polygon is a calculated value. The
fact that they are wrong, however slightly, points up a “problem” for those users who want exact
answers: computers cannot be counted upon to give them.
11 You added the Samples toolbox earlier in this chapter, in Exercise 4-3.