Digital Elevation Data

How to acquire NED, SRTM and Orthoimagery data from the USGS website
How to get them into ARCGIS and change the projection (e.g. to UTM)
How to produce a hillshade map
How to produce a simple (but nice!) map in ARCGIS

How to acquire NED, SRTM and Orthoimagery data from the USGS website

This is the home page of the the USGS Seamless Data Distribution System. The website provides all kinds of data, including National Elevation Data (NED), Shuttle Radar Topography Mission (SRTM) data of various spatial resolutions and ortho-rectified air photos (e.g. DOQQ). A pull-down menu on this site lists all the data sets available. Note also their tutorial: http://gisdata.usgs.gov/Website/Seamless/tutorial.asp
As mentioned, the NED are available in different resolutions. To get detailed information about the individual data sets, just go to the List of Products menu (here the 1arc second NED was chosen) and another window will open (next figure).
Here is all the information about the various data sets. In order to download them however, you have to go back to the main page (next figure).
Depending on the data you are interested in (national or international) you can select one of the two options View and download United States data View and download International data For our example, we use data from the US -one reason being the excellent spatial coverage and quality of the data. Also, some data sets are only available for the US (such as the NED...).
Quite often it takes a little while to open this site -so try to be patient. As you can see on the right side of the map, there are two pull-down menus, right below the zoom bar. The first, called Display lets you choose which data you want to display. In the second one, you can select the data you want to Download. In both cases you just have to check the respective boxes in order to either display or download the data set. Right now the GTOPP60 Color Shaded Relief is checked and therefore displayed. On the left side of the map are tools that you will use to Zoom and to define an area that you want to get i.e. Download data from.
It is helpful to zoom in to the area of interest. An easy way to do it, is to select the magnifying glass on the top left of the map (is highlighted in red). Then just left-click on the map (hold the button) and define the area you want to zoom into -here it is Arizona. Depending on the area you are looking at, there may be a couple of data sets (e.g. elevation data) available. Normally, you would tend to get the highest resolution data, however, this might be impractical and just unnecessary, depending on the purpose that the data will serve. and the spatial coverage your are looking for. For our example here, we will zoom into Arizona and will obtain a 1arcsec NED and SRTM data to compare their differences. Remember that the 1/9arc sec data set will be 81 times larger then the 1arc sec data set, covering the same area!
Zoom in a little bit more => to South Mountain and surrounding area
So, pick a data set that fits your purposes best. In order to download data, you have to do these two things: A) Change from the Zoom-tool to the Download-tool (both can be found on the left side of the map) -just select the Define downloaded area button. B) On the right side of the map, you have been in a pull-down menu called Display; to the left of it, it says Download =>select that menu and select in the Elevation option the data set you want to download (here we use the 1arcsec NED and SRTM Finish 1arcsec). Then switch to the Orthoimagery and select the data of you choice (we used 1m B&W DOQQ UTM Zone 12). Now you can go to the map and define the area you want to download -left-click and open a window.
When you successfully defined the area of interest and the data your are interested in, this window will open which tells you a couple of things about the data you are about to get (the projection e.g. WGS84, output format, spatial extent, and such). As you may see, depending on the resolution and spatial coverage, you might have to download a number of files. Just click at the download button and another window will open telling you about the status of your request. That may take some time! If it is done, you merely have to save the data and that's it!

It may well be that the pop-up blocker on your computer doesn't allow you to down the file. In this case you have to change the internet options to make the USGS-site a trusted site. Just follow these screen shots (from left to right) to accept the USGS-site as a trusted site so that you can start to download the data.

In case you have trouble with the downloads: sample data

1 arc second Shuttle Radar Topography Mission (SRTM) DEM: 58195259.zip

1 arc second National Elevation Dataset (NED) DEM: NED_02446943.zip

Digital OrthoPhotoQuaterQuad: 27431447.zip

How to get them into ARCGIS and change the projection (e.g. to UTM)

Unpack zipped files that you downloaded; then open ARCMAP and the following window will open

Push the Add data button and the following window will open. Use the Connect to folder button to browse to the location where you saved the (unzipped) data. When you press OK, you will see the folder that contains the DEM file in the 'Add data" window. Double-click it, select the file and add it.

Now the DEM is in ARC!

How to change the projection (e.g. to UTM)

Before we do that, it is helpful to know in which UTM zone your data are...
The following website UTM Grid Zones of the World compiled by Alan Morton features a map that shows all the UTM zones.On the bottom of the site is a little application (Transverse Mercatr Calculator) that allows you to compute the UTM zone, for given latitude/longitude values.
For our example, the area lies in UTM zone 12N
Now that you know the zone, click the ARC toolbox icon to open it.In the menu that opens (see figure below) go to Data Management Tools => Projections and Transformations => Raster => Project Raster (double-click).

In the window that will open (figure below), browse for the input raster (the DEM you just downloaded).
The next thing you have to do is to pick the output coordinate system. As you may have seen, the DOQQ's (air photos) are already in UTM coordinates, using the NAD83 spheroid. Because we will bring both data sets together during next class (and both should be in the same coordinates) we will use the specifications of the DOQQ's => NAD 83, UTM 12N
Click the Select button and go to Projected coordinate system to find your coordinate system (UTM => NAD 1983 => NAD 1983 UTM Zone 12N.prj)

You may find in the CITRIX environment ArcGIS9.2 that the projection interface looks like this:

You should XY coordinate system and select a projected coordinate system.

In some cases when the datum changes during projection (NAD83-WGS83 in the case of the SRTM data), you may need to specify the Geographic Transformation. You will know if there is a green dot next to the cell. If you do, just choose the top of the list of transformations.
When ARC has finished transforming the data it will create a new layer containing the projected data. Now, would be the time to remove the old DEM -actually, it wasn't necessary to add the original data in the first place -generally you can (and should) skip that step.
We will repeat the same procedure with the other elevation data.
You should also make sure that the project itself is using the same coordinate system! Right-click on Layers => Properties => Coordinate System => then it is the same step as before i.e. you can browse for the coordinate system => again, for our example, use NAD 1983 UTM Zone 12N.prj (figure above).

How to produce a hillshade map

In order to produce the hillshade map, you will need use the Spatial Analyst tool. If you can't find it in your tool bar: Go to View => Toolbars and check the Spatial Analyst. Now you should find it in the toolbar
Also, make sure that you have access to some of the essential Extensions: Go to Tools => Extensions and check the 3D Analyst and Spatial Analyst.
Now open the pull-down menu Spatial Analyst => Surface Analysis => Hillshade

In the window that will open (figure below), you can choose the DEM (Input surface) from which you want to produce the hillshade map. The following window gives you a couple of options:
- You may change the Azimuth and Altitude of the 'sun' that illuminates the region
- Check the 'Model shadows' button
- You may exaggerate the elevations by the z-factor -this is especially helpful in flat, low-relief areas.
- The Output cell size tells you the size (in m) of each cell; in our example, the ~28m corresponds to a 30m spatial resolution ( i.e. 1arcsec)
- You may want to save the hillshade map (Output raster)...

The following figure shows the two hillshade maps. The left was made from the NED data, the right one from the SRTM, both had the same spatial resolution (1arcsec).

As you can see, both data sets show some artifacts. Generally these artifacts are closely related to the method that was used to record i.e. digitize the elevation data.
The next thing that we will do, is finding out how big the differences between the two are.
For that, we use the Raster Calculator.
Go to the pull-down menu Spatial Analyst => Raster Calculator
What we will do now, is subtract one DEM from the other: double-click a name to select it; try to avoid typing things in the Raster calculator -this tool is very sensitive when it comes to space...
When you are done => Evaluate.

The raster calculator produce a new map with the differences between the two data sets. It compute that value only in regions where both data sets overlap (no values calculated where the SRTM shows 'holes')
As you can see the differences can be quite large (range of ~45m)! I have to point out that -at the time when the DEM's were downloaded from Seamless- the SRTM data contained a half-pixel shift that we didn't correct for -despite systematic errors that are linked to the method the data were produced this will surely have an effect as well..

How to produce a simple (but nice!) map in ARCGIS

For that we need one of the DEM's and a hill shade map. I chose to use the NED-based material (because it is more complete then the SRTM) => remove the other layers.
As in all decent programs that utilize layers, it is possible to rearrange the position of layers in the stack. We want the DEM to be ontop of the hillshade.
Now, right-click the DEM and select Properties. The Layer Properties window opens => Go to Symbology and change the Color Ramp to one of your liking. After that, go to Display and change the Transparency of the layer (we used 50%) => press OK

Now we want to print that beauty of a map.
Change from Data View to Layout view (below the map, on the left side of the lower scrollbar are 4 buttons, the first one gets you into the Data view, the second into Layout view)
Now you can see you map on a virtual sheet of paper (you can change it's size and orientation: File => Page and Print Setup)
We want to input a scale bar and a legend: Insert=> Scale bar => pick one of your liking; Insert => Legend => the following window (figure below) opens. On the left are all the layers that you have in your project, on the right are the ones that you want to be on the map -there is no need for the hill shade to be on the map so we remove it (see mouse). the click next. A number of other windows will open, where you can enter the legend name and fine-tune the layout of the legend.

Next, you will want to change the name of the explanation to anything but Legend. If you have only one item, like topography, then you can call it "Elevation (m)". If you have more, then you should probably call it "Explanation":

That will get you an ok looking Explanation, but you can do better. Right click on the legend itself:

Then click the Items tab and the Style button:

Then click on the Properties button:

And unselect "Show Layer Name" and "Show Heading"--You can experiment with these to see if you get what you want:

Finally, you need to say ok to all of the legend formatting and go back to your main screen of ArcMap. You will see that the Explanation looks pretty good, but the labels are not well formatted. To fix that, you go to the Layer Control on the left of your ArcMap screen and right click the DEM and then select the Symbology Tab. You will see that under Label for the elevations, you can actually edit those numbers and round them up or down or at least truncate their significant figures as you see fit: