VTP Software Tutorials

Tutorial 2: Roads and a Large River

This tutorial will walk you through adding some culture to a terrain, using USGS data sources.  The area is Chattanooga, Tennessee. 

Step 1: Locate the Elevation Data

We'll need DEM (for elevation) and DLG (for the roads).  These are free USGS data, so we look in the usual USGS data sources, as well as looking under "Tennessee" on the USA page.  In general, if you don't find the data you need from those links, it's also often useful to Google for terms like "Tennessee DEM" or "Tennessee data download".

In this case, we rapidly find the site Tennessee Federal GIS Data Server, which has the first data we'll need: 10-meter DEM.  But, notice they are listed by their USGS quad name.  Which quads do we need for Chattanooga?  This is when it's useful to consult a quad map.  You'll find one in SHP format on that GIS Data Server site.  View it in any GIS program, and you'll see something like this in the Chattanooga area:

This tells us the quad names around our area of interest, so we can download the corresponding 10m DEMs: Chattanooga, East-Chattanooga,  Fairmount, and Daisy.

You don't even have to unzip them, you can drag and drop them directly into VTBuilder to see this:

You can zoom in and see how detailed it is.  That's some quite lovely, dramatic elevation.

Step 2: Consider the Projection

Look in the Status Bar, and you'll see the map projection has the Datum NAD27.  To make a long subject short, this is a very old standard used in the USA, from 1927 in fact.  To make things easier for us later, it is better to use NAD83, which is interoperable with WGS84, the global standard.  Some USGS data will come to you already in NAD83.  These are not, so we'll convert, which is easy.

Menu Layer: Convert Projection.  Change the Datum from North American Datum 1927 to North American Datum 1983.  Press OK.  Notice that the Status Bar now says NAD83.

Step 3: Locate the Road Data

Unless you can find some good vector from a local source, there is always USGS DLG.  In this case, the local source is Hamilton County GIS Dept. and they want lots of money, so that's out.  They do have a reasonable view-only map server, which can at least be a useful visual reference while putting the data together:

We can find the vector data we need on the page for Tennessee DLG.  Again, they are listed by quad name, and now we know the four quads we're interested in.  In each folder there are several kinds of DLG available: boundaries, hydrography, hypsography, transportation.  In this case we want roads, so that's in the transportation folder, which is further divided into three files (MT, RD, RR).  From reading about DLG, you know or can guess that RD has the roads.  Download the RD file for each of the four quads ( Chattanooga, East-Chattanooga,  Fairmount, and Daisy ).

Now you have four files which end in ".RD.sdts.tar.gz"  Again, you don't have to unzip them, just drop them into VTBuilder.  You should see some messages about projection difference (the DEMs are now NAD83, but the DLGs are NAD27).  Simply press Yes to allow the automatic conversion.

Zoom in and you can see there is lots of good road detail:

Step 4: Crop and Save the Desired Area

From Tutorial 1, you already know how to sample and save elevation.  The only difference here is that there are multiple input DEMs.  This is no problem, as VTBuilder will automatically merge them together and fix the seams for you.

Let's pick an area which gets downtown and uptown, and makes a good-size 2k elevation map at 10m resolution.  10 meters * 2k samples = 20480, so that's the size area we want.  Menu Area Tool: Numeric Values.  Paste the following into Origin and Size and press OK:

(648330, 3874650), ( 20480, 20480)

Export the elevation using menu Area Tool: Sample Elevation.  Send it to a file called chattanooga.bt in the Elevation folder of your data path.  Be sure to check the "Floating Point" option because 10m DEMs have great sub-meter vertical precision, and it would be a shame to lose it:

Step 5: Crop and Export Roads

Road data is a little different.  They are in four different layers, so the first thing we do is select any of your road layers, then menu Layer: Flatten Layers.  This joins them into a single layer.  Select the new layer, then use the option Select Roads to drag a rectangle around the roads we want (those inside the area of interest).  Menu Edit: Invert Selection will highlight just the roads around the edge we don't want:

Menu Edit: Delete deletes those roads, leaving us with just the roads we want.  One last step is menu Roads: Clean Roadmap.  This will tie up the loose ends caused by joining the multiple inputs.  Finally save your road layer as chattanooga.rmf in the RoadData folder on your data path.

Step 6: View in Enviro

Follow the steps you learned in Tutorial 1 to use Enviro to make a new terrain and set chattanooga.bt as the elevation file.  Under Terrain Properties:

Launch Terrain View and you'll get a view something like this:

It's nice, but notice how the river looks like a flat green piece of land.  We can fix that.

Step 7: Adding River Depth

The VTP software doesn't have full support for lakes and rivers, but it does have an Ocean Plane, which is simply a flat horizontal plane.  If you have a large river or lake in which all the water is at the same elevation, you can use the Ocean Plane for it.

In our elevation source data (USGS DEM) they have set the height value of the river to the elevation of its surface.  We don't actually know the depth of the river, but we can supply our own artificial depth.

Go back to VTBuilder and load chattanooga.bt.  Hold your cursor over any point on the river, and the status bar shows the elevation is exactly 193.20 meters.  In reality, most river's surface elevation decreases somewhat over its course, but a DEM (which the USGS derives from contour maps) can't accurately model the change.  For a short section of a larger river, or for a lake, treating it as one flat surface is a very reasonable approximation.

We're going to remove all the heixels on the surface of the river, and replace them with lower values.  Menu Elevation: Remove Elevation Range and give the values: 0 193.21.  That will remove everything up and including the river surface.  It will look something like this, with the removed elevation either red or transparent, depending on your View settings:

The river area is now undefined ("unknown"), all the way up to a certain feature which happens to be the "Chickamauga Dam".  To make the river 10 meters deep, menu Elevation: Set Unknown Area to 182.3.  Save Layer As... chattanooga2.bt.

Back in Enviro, make the following changes to your terrain properties:

Now the river has depth with a pretty blue water surface:

This approach should work as well for other large flat rivers (like the Seine flowing through Paris, the Yangtze (Chang Jiang) flowing past Nanjing), or a great many other places, as long as detailed elevation data is available.

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