The objectives of this wiki page is to explain how to 1) obtain the raw elevation data from which elevation contours can be generated from, 2) create these contours and 3) show how to symbolize/stylize and view the contours in Google Earth.

Contour lines are 2D lines of equal elevation drawn on a map (see Figure 10-0). Thus walking on a contour line is similar to walking along the beach, it would be at a constant elevation, no change in elevation, not moving up or down hill. In this case the shore line is a contour elevation of zero. The space between the contour lines is known as the contour interval. Contour lines should never touch but there are exceptions such as a mountain cliff or other type of overhang. Contours that form a “V” shape are most likely a natural drainage feature such as a stream or wash. The bottom of the “V” points upstream/uphill. The closer the contours are to one another, the steeper the surface. This would be the case in mountainous areas that exist in Nevada USA. Conversely, the further the spacing between contours, the flatter the surface. This is the case with plains and large fields like in Florida USA and Louisiana USA. For large areas (say less than 40 acres), contours are usually created from USGS Digital Elevation Models (DEM). For smaller areas (say greater than 40 acres), 3D points are measured in the field by a land surveyor. These points are then imported into a software application such as Autodesk Civil 3D and then used to create a digital elevation model/surface known as a Triangular Irregular Network (TIN). This chapter will only focus on creating contours from DEMs.

Figure 10-0 shows a raster image/graphic depicting the Las Vegas Wash in Las Vegas Nevada. The graphics is from the United States Geological Survey (USGS) topographic maps, created by ESRI and made available as USA Topo Maps to be viewed in ESRI ArcMap as a basemap layer. Since this is just a graphic, the end user cannot select any of the contour lines (vectors) to display the elevation. Showing how to create these contour vectors is the purpose of this wiki.

Below is a detailed procedure on how to generate elevation contours from the freely available USGS DEM data and display those contours in Google Earth. The elevation contours assist the civil engineer in evaluating many characteristics of the site, including determining the existing path followed by storm water surface runoff. Since the DEM data is a snapshot in time, any recent development involving cuts or fills on the site might not be reflected in the elevations. The software applications needed for this exercise are: 1) ESRI ArcMap, 2) ESRI Spatial Analyst Extension, and 3) Google Earth. A license must be obtained to use the ESRI software. A discussion of how to obtain the license is presented at the end of the chapter.

Before beginning, the latitude and longitude values of the site must be known to the nearest second. First, the USGS data site must be accessed at http://gisdata.usgs.gov. The complete site address is shown in Figure 10-1 below.

• Use the USGS NED 1 arc second and NED 1/3 arc second Download Tool NED 1/3 arc second (10 meters) to define an area of interest. The example shown below and throughout the chapter is for Clark County Nevada, which contains the city of Las Vegas.
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Download the ArcGrid format of the National Elevation Dataset (1/3 arc second). The file size for the example is about 350MB for each DEM and takes about 11 minutes to download. Clark County Nevada comprises of four DEMs, so the total size is about 1.3 GB compressed. The screen shown below will appear, confirming the values of longitude and latitude that have been selected.

• Recommend downloading the ArcGrid format of the National Elevation Dataset (1/3 arc second). File size is about 350MB for each DEM and took about 11 minutes to download. Clark County Nevada comprises of four DEMs, so total size is about 1.3 GB compressed.
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• Unzip/extract the four files that represent the area you wish. For the example site, Clark County NV will create a folder named n37w116 and an ESRI ArcGrid file named grdn37w116_13 as shown in Figure 10-3 below (the other three files will have different locations in the file names, such as n36w115 as shown in the figures). The naming convention is grd = ESRI ArcGrid format, n37 = latitude north of the equator, w116 = longitude west of the Greenwich Prime Meridian, and 13 = one third arc second, which is approximately a raster cell size of 10 meters (see Metadata on 1/3-Arc Second National Elevation Dataset, http://extract.cr.usgs.gov/distmeta/servlet/gov.usgs.edc.MetaBuilder?TYPE=HTML&DATASET=NED13). View the files using Windows Explorer to verify that they have been unzipped.
• Unzip/extract the four files that represent Clark County NV. This will create a folder like n37w116 and an ESRI ArcGrid name like grdn37w116_13. The file naming convention is:
  • grd = ESRI ArcGrid format
  • n37 = 37° north of the equator, latitude also known as the y value in grid coordinates
  • w116 = 116° west of the Greenwich Prime Meridian, longitude also known as the x value in grid coordinates.
  • _13 = 1/3 arc second, which is approximately a raster cell size of 10 meters (Metadata on 1/3-Arc Second National Elevation Dataset)
• Viewing the files using Windows Explorer
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• Viewing the files using ESRI ArcCatalog
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• Open ESRI ArcMap and use the <b>Add Data</b> tool to import the DEMs
  • Open ESRI ArcMap and import the DEMs data. When adding the DEM, ESRI ArcMap will open a dialog box asking if you want to create pyramids to improve the speed in ArcMap when zooming in/out (see Figure 10-5). It is NOT recommended to use the “build pyramids” option on the individual DEMs since it takes about 30 minutes to complete and when mosaicking the DEMs together a pyramid raster will be created then. Pyramids are automatically created once the DEMs are mosaicked together by using the Mosaic To New Raster tool in ArcToolbox. Figure 10-6 shows a single DEM displayed with a grayscale gradient from black (lowest elevations) to white (highest elevations). Within ESRI ArcMap, the Identify tool can be used to query the DEM with a mouse click to see the elevation of clicked grid cell. When done adding all four DEMs to ArcMap, your map display window should look similar to Figure 10-7.
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  • ArcMap will show a single DEM displayed by default with a grayscale color gradient from black (lowest elevations) to white (highest elevations). On a side note, To see the elevation values of the a single DEM cell, just use the Identify tool.
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  • When done adding all four DEMs to ArcMap, your map display window should look similar to Figure 10-7.
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• If your project site fits within a single DEM boundary then this step can be skipped. Often your site will overlap two or more DEMs. So the procedure to join/merge these adjacent DEMs together is known as a Mosaic. Remember, the objective is to create elevation contours of the site. To ensure continuous contours it is best to work with a single elevation model, otherwise if there are separate elevation models (DEMs), then the contours typically won’t match along the boundary edges. In the case of this example for Clark County Nevada, the four DEMs will be mosaicked into a single DEM using the ESRI Spatial Analyst extension - Mosaic To New Raster tool (Figure 10-8). This tool is found by doing a search within ArcMap and typing Mosaic To New Raster. The tool uses the four individual DEMs that represent Clark County NV as input. Then enter the output folder location, in this case C:downloadsNED10mCC. Create a name for the mosaicked grid, that is the single seamless grid for Clark County. In this case the ArcGrid name will be NEDDEM10mCC. The spatial reference for the raster geographic reference system (e.g. Longitude-x and Latitude-y) based on the horizontal datum is known as the North American Datum of 1983 (NAD83). Figure 10-8 shows pertinent values highlighted in yellow. In addition, Figure 10-9 is an excerpt from the DEM metadata, which is included in the download of DEMs.

In order to complete the box shown in Figure 10-8 it is important to know that all NED data are distributed in geographic coordinates in units of decimal degrees, and in conformance with the North American Datum of 1983 (NAD 83). All elevation values are provided in units of meters, and are referenced to the North American Vertical Datum of 1988 (NAVD 88) over the conterminous United States. (metadata.txt that is included with the USGS DEM download). Since the DEMs are single band images which have pixel values that represent elevations, enter 1 in the number of bands. It is recommended that background geoprocessing be turned off within ArcMap, in the Geoprocessing Options found under the Geoprocessing menu.

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• Before you run Mosaic To New Raster, recommend turning off background geoprocessing, ArcMap file Menu ? Geoprocessing ? Geoprocessing Options so you can see the progress of the tool.
• Mosaic the four DEMs into a single DEM using the ESRI Spatial Analyst extension - <b>Mosaic To New Raster</b> tool
• This tool is found by doing a search within ArcMap and typing in the tool name, that is Mosaic To New Raster.
• The tool with use the four individual DEMs that represent Clark County NV as input.
• Then enter the output folder location, in this case C:downloadsNED10mCC.
• Provide a name of the mosaicked grid, that is the single seamless grid for Clark County. In this case the ArcGrid name will be NEDDEM10mCC.
• The spatial reference for the raster a geographic reference system (e.g. Longitude-x and Latitude-y) based on the horizontal datum known as NAD83, see Figure 10-8 which pertient values highlighted in yellow. In addition, here is an excerpt from the DEM metadata:
  • “All NED data are distributed in geographic coordinates in units of decimal degrees, and in conformance with the North American Datum of 1983 (NAD 83). All elevation values are provided in units of meters, and are referenced to the North American Vertical Datum of 1988 (NAVD 88) over the conterminous United States.” (metadata.txt that is included with the USGS DEM download).
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• Once you click OK on Figure 10-8 dialog box, it will take about 10 minutes to complete the process to generate a single seamless DEM for Clark County. Figure 10-9 shows the progress of the mosaic tool.
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• After DEMs have been mosaicked, ArcMap will automatically add the new seamless DEM and it should look similar to Figure 10-10.
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• Step 4 clips the edges of the mosaic so that only the specific site is shown. If there is no file of boundaries, then skip Step 4.To improve performance and undertake analyses to create the elevation contours using ArcMap, it is recommended to exclude those areas that do not impact the project site. Often in civil engineering when design is taking place on a site, a distance of only about 100 feet beyond the project site is shown on the improvement plans. So, generating elevation contours beyond the 100 foot buffer has very limited value in the final design drawings. Now, in determining what the storm water impact will be for the site, elevation contours may be needed well beyond the 100 foot buffer. In this case, the upstream area is critical to the analysis of storm water impact. Determining the project boundary then requires engineering judgment. If, when undertaking a watershed/basin analysis of the runoff generated upstream of the project site, it is seen that the basin beyond the contours is contributing a large amount of runoff, then the project boundary will need to be expanded to include the relevant area. If the area is very big, and will require generating contours well beyond the upstream basins, then computer performance will suffer. Google Earth is limited with respect to the number of vectors (elevation contours) it can display. So, if the dataset is too large, it might not be possible to view it using Google Earth. In general, it is better to have more data than not enough, but this limitation must be kept in mind when determining the project boundary. To undertake Step 4, it is necessary to download a shapefile of the project boundary. For example, the boundaries of the site in this study are in the file cee110bndy.zip.
• To increase the performance and analizies done by ArcMap in creating the elevation contours, it is recommended to exclude those areas that do not impact the project site. Often in civil engineering design of a site, only about 100 feet beyond the project site is shown on the improvement plans. So, generating elevation contours beyond the 100 foot buffer has very limited value. Now, in the case of determine what the storm water impact is for the site, elevation contours are need well beyond the 100 foot butter. In this case, the upstream area is critical to the analyses of the storm water impact. Determining the project boundary is then an engineering judgement. If, when your doing your watershed/basin analysis of the upstream runoff on the elevation contours you created in this exercise, you see the basin is contributing beyond the contours, then you will need to expand the project boundary. If on the case the area is too big, that is generate contours well beyond the upstream basins, then your computer proformance will suffer. Also, Google Earth is limited on the amount of vectors (elevation contours) it can display. So, if you dataset is too large, you might not be able to view in Google Earth. In general, it is better to have more data then not enough, but just keep this in discussion in mind when determining your project boundary.
• download a shapefile of the project boundary, that is the onsite limits, example cee110bndy.zip. Then just extract the shapefile using Windows Explorer or download and install 7-zip.
• Use the ESRI ArcMap Add Data command to view the project boundary. If a file boundary file exists, it should look similar to Figure 10-12 when viewed, where the map display is zoomed to the extents/limits of the project boundary within ArcMap. This map can be zoomed to its limits by use of the Zoom command in ArcMap. This boundary file is used to clip the raster so that what is displayed is the site.
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• This extent will be used to clip the raster and thus create a subset/smaller elevation model. Figure 10-13 shows the procedure to clip the raster. Following is a discussion on the procedure.
  • Within the ArcMap Table of Contents panel shown above in the left of Figure 10-12, right click on the NEDDEM10mCC DEM raster layer and select Data ? Export Data. Click the Data Frame (Current) radio button to use the current map display in ArcMap as the boundary used to clip the mosaicked DEMs. Within the ArcMap Table of Contents panel, right click on the NEDDEM10mCC DEM raster layer and select Data ? Export Data. A new dialog box/window should open with the title Export Raster Data. Click the Data Frame (Current) radio button (highlighted in yellow in Figure 10-13) to use the current map display in ArcMap (Figure 1-12) as the boundary used to clip the mosaicked DEMs. You will notice at the right side on Figure 10-13 the DEM cell size is approximately10 meters (9.259 meters to be exact).
  • It is recommended that the mosaicked and clipped site map be exported to TIFF for compatibility with Google Earth and other software applications. This is done by clicking on the drop down menu on the right side of Figure 10-13 near the bottom. The TIFF file must be named, for example “ProjBndyDEM.tif.” When done entering the file name, just click the Save button to create the ProjBndyDEM.tif clipped DEM.
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• The USGS DEMs have elevation values that are captured approximately every 10 meters. If elevation contour lines are created directly from this data, the contours might appear jagged. Often, the contours need to be smoothed so they reflect a natural weathering of the land. It is suggested that the DEM raster image be smoothed first. Then the resulting contours will not appear jagged. A filter will be applied to the DEM elevation surface by replacing each cell value with the average/mean value of its surrounding cells. In other words, a 3×3 matrix will be made and the average elevation of those 9 values will be assigned to the given center cell. This is done for the entire DEM and results in a loss of elevation details but produces smoother elevation contours. A quote from the ESRI Help Center on How Contouring Works follows:
  • “The easiest smoothing approach would be preprocess the input raster with the Focal Statistics tool, using the Mean statistic.” (ArcGIS Resource Center - Desktop 10 - How Contouring Works)
• The procedure to smooth the DEM is as follows:
  • Within ArcMap, activate the Spatial Analyst extension (if not already done) by clicking Extensions under the Customize menu. Then check the Spatial Analyst checkbox to activate.
  • Before you run the Focal Statistics tool, ensure you have a geodatabase to store the results. The output of the Focal Statistics tool must be saved in an existing GeoDatabase. If you don't have one, you can easily create one using ArcCatalog ? New ? Personal Geodatabase.
  • To run the average filter in ESRI ArcMap, click the Search collapsible panel on the right side of the application window. Click the Tools link and then enter <b>Focal Statistics</b> (Search ? Tools ? Focal Statistics). Fill out the dialog box/window similar to Figure 10-14. When completed, will have a modified DEM where the elevations values have been adjusted based on the average neighboring cells. This will procedure smoother contour vectors to eventually be displayed in Google Earth.
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• Up to this point, there are no contour lines on the map. This step adds the contour lines. The USGS DEMs have elevation values in units of meters. Typically civil design in the United States uses units of feet. There are two different options one can take to perform the unit conversion. Option A is to convert each value of the DEM to feet which results in the creation of another DEM surface. This presents a problem only if there is limited storage space on the computer workstation. Option B is to just scale the elevation values of the output contours. Then a second DEM grid is not created. So if the only objective is to create contours, then option B is recommended.
• Since the DEM grid cell size is approximately 10 meters, recommend creating contours every 10 meters. It is customary to use elevation values of 25 feet instead of 30 feet as the conversion of 10 meters to approximately 30 feet would produce.

• Ensure the Spatial Analyst extension is enabled in ArcMap by clicking Customize ? Extensions and then checking Spatial Analyst. Also recommend during off the background processing in ArcMap by clicking Geoprocessing ? Geoprocessing Options and then under Background Processing, uncheck Enable. Within ArcToolbox, do a search for the Raster Calculator (Toolboxes ? System Toolboxes ? Spatial Analyst Tools ? Map Algebra ? Raster Calculator) Map Algebra tool.
• To convert the elevation values from meters to feet, use the converstion of 1 meter = 3.2808 feet. In the Raster Calculator, the input raster is NEDDEM10mmCC and the output raster is NEDDEM10mCCft. Each cell value will be multiplied by 3.2808 and thus convert from meters to feet. It will take about 2-3 minutes of process time to convert the units and create the contours, which still need to be completed. The conversion factor to be inserted (Figure 10-15) is 1 meter = 3.2808 feet, which is inserted as *3.2808 in the box using the keypad.
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• For more information on the Raster Calculator see ArcGIS Resource Center - Desktop 10 - Multiplication Operator

• An alternative to create a new raster DEM surface with all the elevation values converted to feet, is to add an optional Z factor to the output contour layer. This is done within the ArcMap &rarr ArcToolbox, and then do a search for the Contour (Spatial Analyst tool). The input raster will be the clipped raster to project limits which also had the elevation values smoothed (ProjBndyDEMSmooth). The output contours must be saved within a geodatabase as the option to save as a shapefile is not available in version 10 of ArcMap. Recommend creating a blank geodatabase called ProjBndyDEM.mdb using ArcCatalog. Name the output polyline features as contours25ft. Use a customary contour interval of 25 feet, keep the default base contour value as 0 and finally enter the Z factor conversion scale of 3.2808. Click OK to create the contours. Remember, the output polyline features will be vector contours at 25 foot intervals.
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• To improve the readability of a topographic map, the elevation contours have a different style/color between the major contours, say every 100 feet and the minor contours, say every 25 feet. To determine if a contour value is a major contour, then it will be evenly divisible by 100, that is a remainder of 0. This calculation can easily be done using the Modulus (also known as the Remainder) operator. With the calculation done, to is an easy thing to then symbolize the contours in ArcMap by assigning a heavier/thicker lineweight and darker color (e.g. black) to the major contours and then a thinner lineweight and/or lighter color (e.g. gray) to the minor contours. What follows next is a discussion on how to symbolize elevation contours.
• Create a new attribute field called <b>minor</b> in the contours25ft polyline feature class. Use a data type of short integer or boolean to store values of 1 (true) or 0 (false) (see Boolean data type). So in the minor field a value of 1 imples, yes this contour is a minor one of 25 feet and a value of 0 imples, no this is not a minor contour but a major contour with values in the 100 feet interval.
• In ArcMap, open the attribute table of the contours25ft polyline feature class. Right click on the newly create minor field and select the Field Calculator command. Turn on the Show Codeblock and then type in the following code:

> x = Contour Mod 100

if x = 0 then

y=0

else

y = 1

end if

* Then in the minor input, enter the variable y. The Field Calculator will loop through all the records/features in the contours25ft polyline feature class. If the contour value Contour is evenly divisible by 100, that is remainder 0, then assign a value of 0 to the y variable, that is false minor contour. Otherwise assign a value of 1 to the y variable, that is true minor contour.

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• Finally, to symbolize the contours into major and minor contours, in ArcMap right click on the contours25ft feature class and select Properties to open the Layer Properties dialog box/window. Click the Symbology tab and then under the Value Field dropdown list, select the minor attribute field. Click Add All Values button. Should see a list with 0 and 1. Change the 0 symbol to a bold black line and the 1 symbol to a light gray line. Click OK and should see the major and minor contours in ArcMap.

Create major contours at 100 ft in black color and minor contours at 25 ft in gray color.

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• Note, when these contours are eventually exported to KML in a step below, the symbolization done in this step will be preserved.

• Once the contours are created, to determine the individual contour value either use the Identify tool and select a contour to the elevation value or alternatively label the contours. Please note, the procedure to create contour labels will NOT be preserved/displayed in Google Earth (which only support point labels and cannot label lines and polygons). So, this step can be skipped if your only interested in contours that can be displayed in Google Earth. Otherwise, this procedure shows how to label contour line using ArcMap.
• Within ArcMap, right click on the contours25ft polyline feature class and select Properties to open the Layer Properties dialog box/window. Click the Labels tab and change the Text String Label Field to Contour. Also in the upper left corner, check the <i>Label features in this layer</i>. Finally click the Placement Properties button to place the contour values on the Line.
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• Contour labels are placed on the contour line. A mask can be applied behind the contour label which will block the underlining contour line. Problem is the mask will also block the view of the underlining DEM or Aerial Photo imagery and draws too much attention to the contour label/value. Recommending ESRI Knowledge Base - HowTo: Mask line features by their labels in ArcMap
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• For more information see Placing labels for contours

• The final step in ArcMap is to export the contours25ft polyline feature class into the native Google Earth format of kml/kmz. As a warning, the Google Earth graphics display is not as robust as that of ArcMap, so if you have too large a dataset of contours lines, it might be easy to view in ArcMap but not in Google Earth. To export the contours to kml, please follow this procedure:
  • Optional - if you want the Google Earth Balloon Descriptor to show the contour elevation instead of the feature ID, within ArcMap, open the Layer Properties of contours25ft, then on the Display tab, change the display expression field value from ID to Contour.
  • Within ArcMap, use the Layer to KML tool (System Toolboxes ? Conversion Tools ? To KML ? Layer to KML) to convert the contours into a format that can be read by Google Earth. The input layer is the contours25ft polyline feature class and the output file is the contours25ft.kmz (note a kmz file is just a zipped/compressed version of the KML file). Finally for the Layer Output Scale, enter a value of 1 to ensure the contours can be displayed in Google Earth at any scale.
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• Note, the Layer to KML only does a 2D projection to lat/long geographic coordinates. If your data is 3D and has elevation units of feet, it might appear to float above the ground because Google Earth uses elevation units of meters.

• To view a KML/KMZ file in Google Earth, either drag and drop the contours25ft.kmz file onto the Google Earth application window or within Google Earth, File ? Open and ensure the file type is KML/KMZ, see Figure 10-22.
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• Remember that Google Earth only has the ability to label points, note line and polygons. So, to determine what the elevation of a contour line is, will need to click on the actual line to view the attributes in the balloon descriptor/info window.Final output should appear similar to Figure 10-23.
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• The final project contour layer can be downloaded from ProjBndy25ftContours.kmz

• As a side note, using Google Earth Pro, one can draw a path/line on the Google Earth terrain layer then right click to display the elevations along that line, known as a profile. Figure 10-24 shows an example profile along the alluvial fan near the Las Vegas Wash.
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The procedure discussed in this example is used to create elevation contours for the project site in Las Vegas Nevada. To adapt to your own project site, just download the appropriate USGS DEMs. Optionally if you have a project boundary to clip DEMs follow Step 4. The ten steps described above create a valuable contour map that can be used when delineating the storm water runoff basins/watersheds that impact your site.

• ArcGIS for Desktop and Extensions Student Trial Software - One-Year Education Use Only
• Google Earth Pro grant licenses

• It is a common civil engineering practice to hire a surveyor to survey the project site boundary and extra 100 feet beyond. For areas outside of the surveyed site, use the USGS DEM data to view the historical offsite basins/watersheds that might impact the project site. The procedure discussed in this example was used to create elevation contours for an entire class project site in Las Vegas, Nevada, using USGS DEMs. To adapt the method to a different project site, the appropriate USGS DEMs and project boundaries (if available) should be downloaded and the steps followed.

• Digital Terrain Modeling: Principles and Methodology by Zhilin Li, Qing Zhu and Chris Gold. Published by CRC Press. ISBN 0-415-32462-9. UNLV Book Stacks Call# GA 139 L5 2005. (pdf)
• Digital Elevation Model (DEM)
• DEM Creation and Analysis
• 27 Ideas for Teaching With Topographic Maps
• USGS DEM
• For more information on the Raster Calculator see ArcGIS Resource Center - Desktop 10 - Multiplication Operator
• Learn Google Earth

• Some end users have expressed interest in using ESRI ArcGlobe instead of Google Earth. Since this is beyond the purpose of this wiki page, I will only cover this briefly. Similar to Google Earth, the vertical exaggeration can be change in ArcGlobe. Figure 10-23 shows a 5x vertical exaggeration. Also the ESRI ArcMap - USGS Topo Map layer can be draped over the 3D relief as shown in Figure 10-24.

Develop Site Date Introduction to Civil Engineering