This tutorial describes how to use DEMview and U.S. Geological Survey Digital Elevation Model (DEM) files to create realistic landscapes in KPT Bryce. The tutorial has three parts:



 DEM to PICT Basics describes how to save a Bryce-sized section of a DEM image.

Bryce DEM Terrains demonstrates accurate DEM terrain scaling in Bryce, and includes tips on exploring DEM terrains in Bryce.

Merging DEM Images describes how to use multiple DEM files in a single Bryce terrain.

What you need for this tutorial



To follow along with the examples here, you'll need:




 KPT Bryce

DEMview

DEMview Sample DEMs (Rocky Mountain Nat'l Park DEM files (40105D7.dem and 40105D8.dem), from the DEMview sample files, also found on CD-ROM included with the The KPT Bryce Book in the folder 'Software:DEMView:Rocky Mountain 7.5 minute DEM: ' )

An image editing program (the examples use Adobe Photoshop 3.0), and

Several megabytes of free disk space.





 I'm assuming you are reasonably familiar with Bryce, and that you have successfully run DEMview at least once to look at a DEM file. Getting familiar with Bryce is mostly a matter of time and experimentation, but the Bryce Explorer's Guide and The KPT Bryce Book by Susan Kitchens are both invaluable references. The DEMview Guide, distributed with DEMview, describes DEM files, where to get them, and how to use DEMview to translate them into Bryce terrains. If you don't have them, go back to the DEMview Introduction and grab 'em.


DEM to PICT Basics

The first step in rendering USGS DEM files as Bryce terrains is converting the DEM data to a gray-to-height image. DEM files are specially formatted text files containing arrays of elevation data. DEMview reads these files and creates gray scale PICT images of the data. Since the Bryce terrain editor likes certain sized gray scale images best, DEMview lets you save Bryce terrain-sized sections of DEM images.

Let's try a simple example: we'll export a 128 x 128 section of a DEM file to use in Bryce.

Saving a selection



First, open 40105D7.dem from the DEMview-SampleDEMs archive. DEMview will read the file and display a gray scale image entitled "Fall River Pass" (tip: DEM images look best if your monitor is set to millions of colors or 256 grays). See the gray strips at the edges of the image? Those are the boundaries of the DEM data. Seven and a half minute DEM files are gridded to UTM cartesian coordinates, not geographical coordinates. That means 7.5-minute DEM quadrangles don't line up with geographical coordinate boundaries. When viewed on a geographical coordinate grid, 7.5-minute DEM quads are slightly skewed. DEMview fills the gaps with a neutral gray color.

Drag a 128x128 square in the window, near the river junction in the southwest corner of the image. The selection indicator in the upper left corner of the window displays the dimension of your selection square as you drag. To move the selection over the river junction, drag the selection square. The cursor changes to a right-pointing arrow when it's over the selection:

Choose "Save Selection As" from DEMview's File menu to save your 128x128 selection as "Fall River Pass junction". DEMview then tells you the proper scaling factor for the terrain in Bryce:

A word about scaling

Elevations in DEM data files for the Continental U.S. range from -86 meters in Death Valley to +4418 meters on Mt. Whitney--a range of 4504 meters. World-wide, the range of elevations is more than twice that: 9243 meters from the shores of the Dead Sea (-395 m) to the peak of Mt. Everest (8848 m). Needless to say, you lose resolution when you try to represent that range of elevations in 256 gray levels. Mapping to 256 gray levels also affects the relative scale of the horizontal and vertical dimensions of the data. When you render the terrain in 3 dimensions, you need to match the horizontal scale of the DEM data to the gray-to-height map vertical scale.

The DEMview Guide goes into the gory details of how height scaling is computed for Bryce terrains. Fortunately, DEMview does the math for you. The dialog you see when you save a PICT or a selection tells you the relative horizontal-to-vertical scale which will accurately render the DEM data in Bryce. Although Bryce doesn't give you fine numeric control over object scaling, it's possible to do reasonably accurate scaling using geometric objects as rulers in Bryce's top view.

This section of the tutorial covered the basics of preparing DEM data for Bryce terrains. No mysteries here: as you've seen, DEMview makes it easy to create gray-to-height images from DEM data. The same concepts apply to 1-degree DEM files. Aside from the obvious difference in scale, 1-degree DEM files also differ in the coordinate system used to grid the data. One-degree DEM files use geographic coordinates, so they don't have gaps at the map boundaries. Also, since most 1-degree DEMs are 1201 by 1201 samples, you can create up to 1024 by 1024 terrain images from them.

In the next section, we'll render Fall River Pass Junction in KPT Bryce.