How Surface Aspect (3D Analyst) works
The Surface Aspect tool creates an output 2D polygon feature class containing polygons that classify an input triangulated irregular network (TIN) or terrain dataset by aspect. Aspect is the direction of steepest downhill slope.
Each surface triangle is classified into an aspect class. By default, aspect is divided into eight, 45-degree-wide classes: north, northeast, east, southeast, south, southwest, west, and northwest. Codes 1 through 8 are used, respectively, to represent these aspect classes. Contiguous triangles belonging to the same class are merged during the formation of output polygons.
The Class Breaks Table parameter is used to define custom aspect classes. Each record in the class breaks table needs to contain two values that are used to represent the aspect range of the class and its corresponding class code. When this argument is not specified, the default classification is used.
Table example break, code 90.0, 1 180.0, 2 270.0, 3 360.0, 4
Note the comma delineation and use of decimals in the first field. Field names are needed but are ignored. The first field represents breaks, and values need to be in decimal form; the second field represents codes, and values must be integers. The table can be either a .txt or. dbf file in a Windows environment and a .dbf file in a UNIX environment.
The name of the attribute field used to record the polygon aspect codes is Aspect Field. Its default value is Aspect Code.