How to Convert Large Geospatial Grids
Various Geospatial elevation grids can be loaded, such as ArcGIS ASCII grids, Surfer grids, ZmapPlus, GeoTIFF, IMG, etc.
These can often be larger than 1K x 1K resolution, in a uniform grid pattern. The following steps are
recommended to achieve an effective conversion to a reasonably sized interactive 3D PDF terrain surface
representation.
When processing typical ArcGIS ASCII Grid Files (.asc), it is very common the horizontal, vertical grid cell size is specified in decimal degrees, latitude-longitude. The actual elevation or depth values, Z, are normally in some other units, such as meters. In addition, such grid tiles may be large, 1800 x 1800 (25 Mb) or greater. The following notes indicate a typical strategy to generate 3D PDF interactive summary view results from such data sources. As a typical example tile, assume tile cell size is 0.00027778 degrees.
Loading Grid, Input/Output Tab
- User File-Open menu, or select Add File, and load your single grid file with .asc extension. The file should not be open in any other software, should be ASCII, with typical ArcGIS grid header lines.
- Set your suitable output directory, where you have write-access, and name your desired output PDF file.
Subsampling
-
On the Gridding Tab, Set Sub-sampling to 4 (to load every 4th row and column only, for 1/16 grid data size in memory). Start with large subsample value to make test conversions. This is highly recommended as a starting conversion, then adjust as needed later.
-
Select Skip Full Grid (to reduce texture memory overheads, colour overlay will be at 1/16 size).
Conversion Tab
- Best results may be obtained with PRC-HCT compression type, which includes terrain grid optimisation.
- Set PRC tolerance 0.000005, to reduce depth numerical truncation which may become visible as surface uneveness.
PDF 3D View Tab
- Set View Scheme to Geospatial, to get North Compass and typical architectural and GIS default views.
- Set Insert Controls for Z Scaling, leave default value at 1.0 Users can change it in PDF later.
- Set Lock Render Mode, if you prefer users not to see underlying grid cell structures.
Scene Axes
- Enable X, Y, Scalebar options for scene ruler.
- Enable Compass symbol to have normal corner axes replaced with a North geospatial indicator.
Geospatial Tab
- Set degree to meter conversion factor to 0.0002 (to make visual exaggeration of depth variations). This factor is internal, for cosmetic purposes. Axes labels and probe values show original data units. This is if data is relatively flat. If you have significant topography, this can be lowered.
- Select Degrees as XY Axes Unit (as ArcGIS XY units are most typically in Lat-Lon decimal degrees).
- Load a colormap file, in .clr or .xml format (optional, to show visual depth variations and enable legend). You can also set exact min-max range on colormap if you prefer some presets to match your normal color contour bands.
Gridding Tab
- Enable TIN representation, set Reduction to 0.95. This will create optimized triangle mesh for efficient PDF generation. (Note for GDAL Interface TIN may not be available for some versions of the software, so Advanced Simplification is advised).
Save Session
- Save your session file frequently, so if conversion fails, takes too long, or runs out of memory, all your previous settings may be recovered and adjusted.
- Start with large sub-sampling values to create small test PDFs, then as you make parameter adjustments, decrease sub-sampling to gain higher fidelity, and larger file sizes.
Given these type of parameters, a 25mb ASCII grid tile can be represented with 320K triangles in a 2 Mb PDF file.