3dprocessing

Creating a Terrain Model

From Point Data

Digitize points. Use the COVER-ID value to encode elevation (or whatever the Z variable might be). Position points to capture changes in Z. There is no need to saturate an area that is not varying with points. Build the coverage when done. Add an item and CALC the COVER-ID into that item.

Create a TIN from the point coverage using the ARCTIN command. Use DESCRIBETIN to verify no concavities in the hull and no flat areas. If insufficient detail or too many flat triangles, densify the point coverage and repeat the process.

Convert your TIN to a grid (= lattice) using the TINLATTICE command. Export your grid for transport to your ArcView system.

Check your model with a contour or 3-D surface to make sure that the representation is correct.

From Line Data

Digitize arcs that correspond to contours of elevation (or whatever). Code the arcs for Z (elevation), this should be as an added item. There is no need to digitize all contours. Focus on areas of change. For example, the top of a ridge and the base of the ridge would be important inflection points. The contours in the ridge side might be unnecessary. Don't forget to enter break points.

Build line topology. Be sure the Z variable is encoded in an added item. Create a TIN using the ARCTIN command. You can adjust the number of points that are used in the TIN with the weed tolerance.

Check your model with a contour or 3-D surface to make sure that the representation is correct. If you need to modify the coverage to densify points, add them as points. You can mix lines and points in the input coverage when creating a TIN. Be certain that the added item is the same for the PAT and AAT files.

From a DEM

Create a lattice from the DEM using the DEMLATTICE command.

Massaging your GRID (Lattice)

Subsetting and making a TIN: Subset the lattice using the VIP command. You can determine how many points comprise your lattice by issuing the DESCRIBELATTICE command. The VIP gives you a point coverage which you use to make a TIN using the ARCTIN command.

If there are rough spots or aberrant points in your lattice, you can remove them with the FILTER command. The LOW pass filter smooths your surface. You can adjust the number of passes the filter makes. Try 3-5 to start. The HIGH pass filter increases the contrast of the model. Be sure to check to see that you are not compromising the quality of the terrain model with these operations.

You can combine lattices using the LATTICEMERGE command. You can clip from a lattice using the LATTICECLIP command.

You can migrate a lattice to ArcView by exporting it as a grid (e.g., EXPORT GRID MYLAT MYLAT) and FTP'ing it your PC. Import the grid using the IMPORT71 utility. You can add the grid to a theme with the spatial analyst extension turned on.

Caution -- Always check the quality of the model you are creating. There is considerable art involved in making a high quality representation of a surface. Never assume it is correct until you have checked it.

Command Summary

ARC/INFO (Unix or NT)

DEMLATTICE -- converts a USGS DEM to a lattice

VIP -- very important points, used to convert a lattice to a point coverage

FILTER -- smoothens a grid (LOW pass filter) or sharpens (HIGH pass filter) the contrast of a surface

LATTICEMERGE -- merge lattices (like APPEND)

LATTICECLIP -- clips lattices (like CLIP)

ARCTIN -- makes a TIN from a point or line coverage. Use CREATETIN for complex surfaces with breaklines.

TINARC -- makes a polygon coverage out of a TIN

DESCRIBELATTICE -- tells you the details of a LATTICE