Terrain Data

Place-Based design projects benefit from an understanding and representation of the local terrain. Terrain data is surprisingly easy to obtain and convert into useful representations that may be used in GIS and also popular CAD and 3d modeling formats. This page discusses the how to obtain georeferenced terrain data for nearly anywhere in the world and georeferencd aerial photography within the United States. A few tools are presented for transforming these raster terrain models into: synthetic shaded relief images; contours for laser cutting; surfaces for 3d visualization, and editable breaklines that serve as the basis of earthwork and grading studies.

Some things you can do with digital elevation models

Digital Elevation Models (DEMs) are a type of raster GIS layer. Raster GIS represents the world as a regular arrangement of locations. In a DEM, each cell has a value corresponding to its elevation. The fact that locations are arranged regularly permits the raster GIS to infer many interesting associations among locations: Which cells are upstream from other cells? Which locations are visible from a given point? Where are the steep slopes? One of the most powerful applications of DEMs is adding synthetic hillshading to maps so that the map reader may see the relationship between terrain and other things you may be mapping.

A Digital Elevation Model (DEM) is a raster of elevation values. Rasters represent the world as regular arrangements of pixels (cells). Rasters lend themselves to systematic analysis of the relationships among places and their properties. For example, a Raster GIS can calculate many useful derivitives of elevation, such as: Slope or Aspect -- the direction of slopes or Visibility -- what is visible from a spot? Synthetic Hillshade calculated from a DEM is a great way to create visualizations of terrain with other semi-transparent themes. DEMs can also be used to create 3-D scenes or to create contour which may be exported to CAD programs.

Download the Sample Dataset

To save time in this tutorial, and to eliminate crazy variables that may cascade from oddities of your particular download, the rest of this tutorial will use the following sample dataset:

• Click here to download the sample dataset expand it into your c:\temp\yourusername folder. Open the ArcMap document dem.mxd that is in the docs folder.

Deeper Reading

... from the GSd GIS Manual

• Fundamentals of Geodesy and Map Projections
• Georeferencing Images explains how to georeference a hi-res image from Google Earth Pro.
• Terrain Modeling Discusses the principles and techniques of terrain modeling.
• Exporting Georeferenced Terrain and Imagery to Sketchup (from here, sketchup pro can export to 3ds or dwg.

... from the ESRI Software documentation

• Using ArcGIS 3d Analyst
• Geoprocessing in ArcGIS

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Finding Free DEM and Land Use Data on the Web

National Elevation Dataset and NLCD, National Land Cover Characterization Dataset are a complete seamless coverage of DEM and Land Cover. These are available for free download via USGS Seamless Data Viewer THis web site makes liberal use of popups which seem to be very difficult to pass through Microsoft Internet Explorer. I recommend that you use Mozilla Firefox. YOu should also make sure that if you have google toolbar, that you shut off its popup blocker for this particular site. YOu will see the USGS Seamless Tutorial linked at the top of the browse page. You should open this in a new window for reference while you are using the browser. I won't bother to repeat the instructions for downloading DEMs here.

Hint: The 1/3 ArcSecond Elevation data are not available for all areas. For this tutorial, I recommend that you download the 1" NED Data -- or use the tutorial dataset, linked above.

References:

• Seamless Data Browser Tutorial

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Important Notes on Handling Grids

The rasters you get are ArcInfo Grid datasets, which are not files, but interrelated directories (or folders) information which have to be handled carefully to avoid rendering them unreadable. The easiest axiom regarding the handling of GRID datasets is Never use ordinary file-system tools to move or rename GRID datasets. Always use ArcCatalog or ome of the wizards from ArcToolbox.

Additional Notes about Naming Grids and Workspaces: Grids, and the folders that contain them, and any folder involved in the path from the disk up (e.g. c:\temp\myfolder\workspace\grid ) must not have names that begin with numbers; nor may any of these have spaces in their names! This is a legacy back to the days when it was impossible to have these things, and for some unkown reason, breaking these rules will cause mysterious and annoying things to happen -- none of them good.

References:

• What is Raster Data
• About the ArcInfo GRID Format
• How Grids and Coverages are Stored The same data structure described here applies to grids

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Choosing a Local Coordinate Referencing System

When you examine the source properties for your new grids, you will see that the spatial referencing system is: GCS North American 1983. It turns out we have to understand what this means. A review of Principles of Spatial Referencing Systems, will explain why an unprojected Geographic Coordinate System (GCS), which reference locations using latitude and longitude, is the only commonly used spatial referencing system that can reference distinct locations anywhere on the planet (which is why it is used by the seamless data browser. However, to use latitude and logitude as coordinates in the cartesian sense, leads to serious misrepresentation of geometry and scale. The relative lengths of a degree of latitude on the face of the planet, may be more than twice the distance spanned by a degree of longitude, depending on where you are measuring. This presents a problem if we want to measure slope, create relief shading, or to compare the relative sizes and shapes of things in a useful way.

Therefore, before we can do anything with digital terrain models, we need to know how to choose an apropriate projected coordinate system for the location we are studying. The simplest guideline to follow in choosing a local coordinate system is to use the appropriate zone of the Universal Transverse Mercator Coordinate System. The tutorial dataset includes a utm.shp layer that will help you determine which UTM zone your location falls inside. If you want to transform your terrain model to units of feet, you may want to consider using one of the State Plane Coordinate System projections. For future reference, you can find shape files in your c:\program files\arcgis\reference systems folder that identify the apropriate UTM and state plane zones for a place.

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Transforming Rasters with Geoprocessing Tools

Digital terrain models are interesting to look at, but more importantly, they can be transformed into other forms of data that are useful for many things. We can calculate slope and aspect, we can make shaded relief that makes the terrain model much more informative graphically. We can even create contours at whatever contour interval we want. Contours are also useful graphically, but also for laser cutting and exchange of three dimensional models with other programs. Probably the most efficient ways to terrain information out of GIS and into a 3d modeling package is as a trangulated surface. This is also quite easy to do!

We will begin by transforming our elevation raster to a shaded relief map and an aspect layer using wizards form the ArcGIS Geoprocessing Toolbox. You will see how to use the individual wizards in the toolbox. A the end of this document, you will see how geoprocessing tools can be chained together to create custom tools that will let us create custom wizards to automate useful tasks!

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Transforming Elevation to Hillshade and Aspect

These two tasks will allow us to introduce the toolbox and a few simple wizards.

References

• Geoprocessing Tools
• Using Tool Dialogs
• An Overview of the Raster Surface Toolset

Making Maps with Terrain Shading

The trick of synthetic relief shading demonstrates a fascinating phenomena of visual communication. Even though the default sun azimuth for the hillshade tool (315 degrees, northwest) may never actually occur at your site, you should use this (or some other northern angle.) Setting your synthetic illumination to be in the north is an important convention because shading works because your eye can easily imagine bumps sticking out of the maps, are being highligeted by the source of light in the room (which is normally overhead.) If the illumination is coming from below, the eye and brain still assume that things are highlighed from below, so the map will appear inside-out.

Check out these examples: Mouse over the words in the left column to change the image

Conventional Illumination This sun angle, 315 degrees, never occurs in Oregon, but the hillshading makes the mountains appear to pop out appropriately. Note that the stream is running through the valley to the river. Natural Illumination This sun angle, 135 degrees, (Southeast) may simulate a mid-morning condition for the site, but the trick of synthetic hillshading makes the valleys, not the mountains appear to pop out! Hillshading is an optical illusion that takes advantage of our assumption that the illumination is from overhead and shadows occur underneath things, not on top of them. Natural Sun Angle, map oriented north-down With this map we can have a natural sun angle -- and effective relief shading -- at the expense of the cartographic convention for keeping north at the top of the map.

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A Warning About Calculating Slope

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The Geoprocessing Environment and Making Contours

Lets open up the slope wizard one more time and click the Environment button and take a look at some of the settings under General Settings Note that properties may be set such as the Output Projection and the Output Extent can give you powerful ways to clip and reproject and perform other operations all in one step. This is very handy. The Output Workspace and the Scratch Workspace let you set up the default location for the outputs of geoprocessing operations. This is particularly useful if you consider that while you may set the environment for one wizard/operation, you may also use Tools->Options->Geoprocessing to set up these environment properties for all of the operations that you do within a single map. For example, we can environment settings to make contours from our original unprojected raster elevation model and convert X, Y and Z units to feet all at the same time!

References

• Overview of Geoprocessing Environment Settings
• The Create Contours tool

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Using Models to Chain Together Series of Geoprocessing Steps

The following parts of this tutorial demonstrate how chains of geoproceesing steps can be put together in models, which are essentially custom tools. In addition to providing a means of pulling several wizards into a single tool. These models allow you to chain the output of one wizard into the next, and also let you fill in the blanks in some of the wizards, and expose other wizard options and environments to the user through a simple interface.

We aren't going to go into all of the techniques of building models here. One of the neat things about models, is if they are designed well, the user doesn't necessarily need to know what is going on inside. Another really good thing about them is that they allow an expert to very suscinctly document a procedure so that users can see exactly what is going on if they are interested. There are some very useful models included with this tutorial, that do things like creating generalized contours and clipped out Triangulated Mesh Models this will be a good occaision to take a look at them and see how they work. We will get to that, but first lets create a simple model to create contours to introduce the basic contents of a model.

References

• What is Geoiprocessing"
• A whirlwind Tour of Geoprocessing
• An Overview of Model Builder
• Focal Statistics
• The Simplify Line Tool (Data Management Toolbox)

Geoprocessing Tip Sheet

• You may want to read this: Setting Up Models for a Collaborative Environment

A One-Step Process for Converting Raster Terrain Models to CAD

Most GSD students think that they have to do everything in CAD -- even though CAD applications are really bad at dealing with very large models and real coordinate systems... Anyway, if you really want to have a terrain model in a CAD package, GIS can help. As you will see from the references above, converting 3d contours to AutoDesk formats involves a couple of steps. Luckily thanks to Geoprocessing models, I have been able to wrap all of the smoothing, a raster turniong it into contours and simplifying them and finally turning it into a DXF file -- all in one step!

References

• Converting GIS Data to CAD
• Export to CAD (Conversion Toolbox)

Turn Raster Terrain Models to Contours with any given contour interval And Excess Vertices that make your CAD meshes needlessly complex can be Removed! and the resulting contours are converted to DXF! YOu may also be interested in the superior meshes that are created directly from raster terrain models These can be made with a tolerance that lets you get the essential information from the terrain model without any excess polygons. these meshes can be exported to Sketchup and in turn exported to dxf of 3ds formats.

Note! if you try to use a function, as is suggested below, and get an error to the effect This Tool is not Licensed you should go to Tools->Extensions and check the boxes next to 3d ANalyst and Spatial Anayst

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Converting to TIN and Exporting to Sketchup

To export your mesh to sketchup, install the sketchup arcgis plugin into ArcMap. If you are on a computer in room 516, this can be accomplished by choosing Tools->Customize and simply check the box for Sketchup 6 Tools. If you need to install the sketchup5 plugin for ArcGIS on your computer, go to the the winapps/sketchup directory, and follow the instructions.

TO export your TIN to sketchup, simply push the sketchup button, and go to the TIN tab, and uncheck the box that says 'Exclude form Export.'

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Visualizing your Terrain Data in 3d

Apologies for this cursory teeaser here, but it will have to do for now. It is surprisingly easy to create visualizations of your terrain data in 3d uusing ArcGlobe. YOu will find useful information about this in the Using ArcGIS 3d Analyst User Guide..

If you open the dem_demo_pt2.mxd in the gis/pbcote/docs folder of your sample dataset, yoiu will see a nice project that makes a nice image overlay using the raster elevation model and contours portrayed in nice colors to indicate land and water, and overlayed transparently on top of the areial photograph. This image was exported as a jpg withg a world file from ArcMap using File->ExportMap. This jpeg will be georeferenced with a world file, and will inherit the projection of the data frame from which it was exported. You wil lhave to use arcCatlog to uypdate the spatial refernce properties of this image before it will open in arcglobe. All of this is worth it, as you will see, if you open the fp_3d.3dd file in gis/pbcote/docs. Consult the 3d Analyst use guide to see how this file can be used to visualize your terrain and buildings in 3d! Hint: movew the DEM layer to the elvation layers group!

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Decifering and Reclassifying your NLCD Data

Each cell in the NLCD dataset is codes with an integer value that represents a class from the Anderson Lancover Coding System. You can download an apropriate lookup table here. Notes on using lookup tables are Here.