Chapter 9

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Isopleth maps, or the three-dimensional equivalent, fishnet displays, should be used for data that are continuous and smoothly changing. Both of these can be displayed within ARCPLOT, but first a surface must be generated from a coverage. The result of this conversion, or any grid layer, can be turned into a isoline coverage and displayed as arcs, or displayed as a fishnet. All of the surface display routines can be used for both grids (lattices) and TINs (Triangulated Irregular Networks).

Polygon to Isoline Conversion

Contours can be used to show continuous, constantly changing data, but if the data are summarized to polygons, isolines must be generated before they can be displayed. POLYCONT.aml accomplishes this. See the discussion of surface calculation on page 84.

POLYCONT <in_cover> <data_item> <out_cover> {contour_interval} {resolution} {i|s|t}

<in_cover>
the coverage to be converted
<data_item>
a numeric data field in <in_cover> that specifies `Z' values
<out_cover>
the output line coverage
{contour interval}
the contour interval of the output coverage--default is 1/10 of the range
{resolution}
resolution of the surface grid--defaults is 1/100 of the small axis in <in_cover>
{i|s|t}
surface calculation method: Idw (default), Spline, or Trend surface.

Polygon to Surface Conversion

The generation of a surface can be accomplished by creating a grid or a TIN. Creation of grids is discussed on page 84. In general, grids may be the better method, unless full use is made of the TIN surface generation capabilities. A simple TIN can be created from ARCPLOT with POLYTIN.aml.

POLYTIN <in_cover> <data_item> <out_tin> {polys|labels}

<in_cover>
the coverage to be converted;
<data_item>
a numeric item in <in_cover> that specifies `Z' values;
<out_tin>
the output surface;
{polys|labels}
whether polygon bounds or label points should be used as the Z reference.