Verify or Select Source Data Resolution to Match Model Scale
Make sure the elevation and image data sources have enough resolution to make a good model.
Select the data source based on the scale range it supports. The following table gives scale ranges for some elevation grid data sources:
Grid Spacing Model Scale Range Public Data Soucre
1 meter data 1" = 70' to 1" = 700' State lidar (OH, CO, UT, others), Mars HRISE and USGS 1 meter lidar.
3 meter data 1" = 200' to 1" = 2000' USGS 3 meter lidar available for 15% of the USGS
10 meter data 1" = 700' to 1" = 7000' USGS 10 meter data, covers the continental US
30 meter data 1" = 2000' to 1" =20000' USGS 30 meter data, ASTER world coverage
For example, if you want to make a 1" = 1000' scale model, use the 3 meter lidar data (if it is available) otherwise use the 10 meter data.
You can also verify the source elevation data set has sufficient resolution by calculating the source grid points per inch in the model. For example, at 1" = 400' scale, 1 meter data has about 122 source data grid points per inch, 3 meter data has about 41, and 10 meter has about 12. Using the 10 meter data results in a smooth low resolution model (144 data points per square inch).
The model is carved at 100 grid points per inch standard. Global Mapper translates the unscaled grid sample interval of the origonal data source to the scaled sample interval at 100 grid points per inch. If the scaled source data has more than 100 grid points per inch, then the exported grid will skip some of the source data points when generating the output grid. If the scaled source data has fewer than 100 data points per inch, then the exported grid is interpolated between the source data points to generate the output grid.
USGS ortho images are available at .15 meter, .3 meter, .6 meter, 1 meter, 1.6 meter resolutions. The image file is laser marked at 500 pixels per inch. The scale for these resolutions which gives 500 pixels per inch is calculated as follows:
.1 meter 1" = 164' .3 meter 1" = 492' .6 meter 1" = 984' 1 meter 1" = 1640' 1.6 meter 1" = 2624'
From this table, if you were making a model at 1" = 400' scale, the .3 meter ortho imagery would be a good match. The .1 meter imagery would be better, if available, but has nearly 10 times the data. The 1 meter image could be used but the 39.37" unscaled pixel width needs to be interpolated to 9.6" (400'/500 = 9.6") sample spacing. This can be done by Global Mapper's raster export command. The exported file has 500 pixels per inch but it's effective resolution is (9.6/39.37)x500 = 121.9 pixels per inch.