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Using and improving surface models built by computer (view source)
Revision as of 13:02, 8 July 2014
, 13:02, 8 July 2014→Techniques for improving maps
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==Techniques for improving maps==
==Techniques for improving maps==
[[file:using-and-improving-surface-models-built-by-computer_fig1.png|thumb|300px|{{figure number|1}}Contour maps of the same surface data. (a) Unconstrained extrapolation into nondata areas. (b) Contours constrained to areas near data.]]
The first surface model and map generated by computer are often unacceptable. Many ways exist to improve the model, some commonly used techniques are described below.
The first surface model and map generated by computer are often unacceptable. Many ways exist to improve the model, some commonly used techniques are described below.
===Do not display (Blank) bad portions of the model===
===Do not display (Blank) bad portions of the model===
Most computer modeling algorithms do not extrapolate adequately. Many mapping systems automatically extrapolate to the map edge, and extrapolations are often not needed. Parameters usually exist for constraining extrapolations to near the data during model construction ([[:file:using-and-improving-surface-models-built-by-computer_fig1.png|Figure 1]]). If these do not exist, then a polygon can be drawn around the data area and the model displayed only inside that polygon. Polygon blanking is also used to blank bad data areas for rush projects when no time exists for making corrections.
Most computer modeling algorithms do not extrapolate adequately. Many mapping systems automatically extrapolate to the map edge, and extrapolations are often not needed. Parameters usually exist for constraining extrapolations to near the data during model construction ([[:file:using-and-improving-surface-models-built-by-computer_fig1.png|Figure 1]]). If these do not exist, then a polygon can be drawn around the data area and the model displayed only inside that polygon. Polygon blanking is also used to blank bad data areas for rush projects when no time exists for making corrections.
===Apply filters to the model===
===Apply filters to the model===
[[file:using-and-improving-surface-models-built-by-computer_fig2.png|thumb|{{figure number|2}}Cross section showing the output from filtering being constrained between models built by shifting the initial surface model up and down slightly.]]
[[file:using-and-improving-surface-models-built-by-computer_fig2.png|thumb|300px|{{figure number|2}}Cross section showing the output from filtering being constrained between models built by shifting the initial surface model up and down slightly.]]
A common problem with computer-generated surface models are surface structures that are not supported by data. That is, the structures are by-products of the surface-modeling algorithm. Filters such as least squares, biharmonic, laplacian, and others can be applied to existing surface models to remove these unsupported structures. Data should be honored while filtering so that data values continue to fall on the correct side of contours.
A common problem with computer-generated surface models are surface structures that are not supported by data. That is, the structures are by-products of the surface-modeling algorithm. Filters such as least squares, biharmonic, laplacian, and others can be applied to existing surface models to remove these unsupported structures. Data should be honored while filtering so that data values continue to fall on the correct side of contours.