Projection Exercise - Downloading and projecting Digital Elevation Models (DEM)

Introduction

In this exercise, you will work with projections, by re-projecting a grid dataset from one projection into another. You will create a map showing both projections together.

Projections - Every spatial dataset is projected in some manner, to transfer the geographic coordinates of latitude and longitude on the three-dimensional earth to X and Y coordinates on a flat piece of paper. When using a dataset, we need to know what projection it is in, so that it can be properly aligned with other datasets. All the data in a GIS data view must be in the same projection in order to line up properly with all the other datasets. In ArcGIS these tasks are often done in the background, so the user does not have to worry about them. ArcGIS will use the projection of the first dataset loaded to set the projection of the view. It will then re-project each dataset on the fly to match that projection, so long as the datasets have a projection defined. If not, it will guess at what projection the data is in.

Defining a Projection - When a dataset is first acquired, it may or may not have a projection defined. If not, the user must define the projection of the dataset. This can be done most easily with the "Define Projection Wizard" tools in the ArcToolbox. Defining a projection for a dataset does not change the dataset, it simply attaches projection information to the data. To check if a dataset has a projection defined, look at it's metadata using ArcCatalog, or load it into a data view in ArcMap, and look at the Data Source information in the Layer Properties Dialog to find projection information.

Changing a Projection - To project a dataset from its present projection into another projection, the location of each point (vertex, grid-cell, etc.) in the dataset must be re-calculated. This is accomplished with the "Project Wizard" in the ArcToolbox.

Data

For this lab you will download the necessary data from the web, as detailed in the instructions.

Software

All of this exercise can be done in the ArcMap, including ArcToolbox, and ArcCatalog modules of the ArcGIS Desktop. 

Each application can be found in the Start menu, under: Start >> All Programs >> ArcGIS, except ArcToolbox, which is accessible from ArcMap.

Downloading a Dataset

(In this part of the exercise, you should be using Mozilla Firefox as your browser. You will not be able to download data in Internet Explorer.)

For this exercise we will use elevation data from the National Elevation Dataset (NED). The NED is a collection of elevation data which covers the entire United States. It has been processed to correct discrepancies, fill holes, match the edges of source datasets, and fill slivers of missing data. More information on the NED can be found at http://ned.usgs.gov//About.asp.

In the past, elevation data was most often available as Digital Elevation Models (DEMs), split by USGS topo quad sheet. In order to see the elevation of a large area, one had to download numerous DEMs and perform combining procedures to stitch them together into a single DEM.

Today, that task has been eliminated since the NED is now available through "The National Map Seamless Data Distribution System" from the USGS. The website for this seamless national map is: http://seamless.usgs.gov. The National Map website makes available a number of datasets, as listed below. The seamless data distribution system allows users to select an area of the country or the world, and to download seamless data for that entire area as one dataset. The system allows for data delivery via free web downloads (with size restrictions), or via CD-ROM media (for a fee).

The National Map Seamless Data Distribution System data sets (as of 9/22/03)

Some areas of the United States are available at 10 meters by 10 meters (100sq meters) while the rest of the country is available at 30 meters by 30 meters. The rest of the world is available at 90 meters by 90 meters. In all cases a single elevation represents the entire area.

Now, let's download a dataset.

Go to The National Map website (http://seamless.usgs.gov), and click on the link to: "View and Order Data Sets - United States Viewer".

An ArcIMS internet mapping site will open up (see image below), with a view of the United States, and the available NED shown. You can use the tools on the left to move around the map, zoom in and out, and select features like within ArcMap. On the right are two lists of data layers. The top one is download layers. The layers selected in this list will be downloaded when you select an area with one of the download tools. The bottom list is the visible layers list, and layers selected from this list will be visible in the viewer. In order to view changes in your selections, you must click the "Refresh Map" button at the bottom of the list (scroll down).

Use the tools to zoom-in and find an area that you are interested in, such as your home, where you like to vacation, or some place you have always wanted to visit. Try to choose an area that has some elevation differences... a flat floodplain won't be a very interesting elevation dataset. Also, if you can, pick an area with features that you will be able to recognize, so that you can see the effects of projection more clearly. The area should be somewhere around 10 to 25 miles on each side, since we don't want too big a dataset for this exercise. Note which state(s) you are getting data from.

Once you have found an area you like, make sure that the NED and NLCD are selected in the Download Layers list.

Use the "Select By Rectangle" Download tool to select the area for which you want to download data.

When you release the button, a new window will appear, and after a moment of processing, you will get a SDDS Request Summary Page containing links to download your selected data. By default, your data should be in ArcGrid format. If you wanted to change the format of your data, the format of the metadata, the datasets selected, or other options, you can use the Modify Data Request Button.

Click on the "Download" link next to the NED dataset. When the data is extracted, select "Save" and save the Zip file it to a new directory in your athena locker (H: drive) directory for the class (i.e. H:/1.963)

Find the file you downloaded using "My Computer" or "Windows Explorer", and right-click it to open the context menu. Select 7-Zip >> Extract Files, and extract the files to your Lab4 folder. (it may automatically create a directory with the file name, which is fine).

Before closing the window with the SDDS Request Summary Page, download and extract the NLCD data set in the same way.

Examine the Data

Open ArcCatalog and navigate to your newly downloaded NED dataset. You will see a grid (the actual data) and a shapefile called METADATA. This file contains polygons depicting the original DEMs from which your dataset was assembled.

Select the grid dataset and click the Metadata tab to view the metadata for the NED grid. Find the section on projections and coordinate systems. What projection is it in?

Close ArcCatalog and open ArcMap with a new, empty map.

Before loading any data, view the Data Frame Properties (under the View menu). Go to the Coordinate System tab. Under Current coordinate system, It should say "No Projection", indicating that no projection is defined for the data view. Close the Data Frame Properties Dialog.

Now, open the Add Data dialog and add in the NED dataset you downloaded.

The dataset should appear on the Layers list.

Go to the new dataset's Layer Properties dialog (Right-click it's name and select Properties from the context menu).

Go to the Source tab and scroll down the top box titled "Data Source" to see the Coordinate System information.

The Coordinate system should show up as "GCS_North_American_1983", as shown below:

Since there is no further projection information, this indicates that the data is in the simplest type of Latitude/Longitude projection, the "Geographic Coordinate System" (GCS). The Datum used is the North American, 1983 datum (NAD83), which is pretty standard today, although some datasets are still in the older 1927 datum (NAD27).

Close the Layer Properties Dialog and go to the Data Frame Properties dialog again. Look at the Coordinate System information again, and note that the Data Frame view has been set to GCS_North_American_1983, to match the data. Close the Data Frame Properties dialog.

Don't forget to Save your work.

Project the Data

Next, we will re-project the data into a new projection.

Open the ArcToolbox from within ArcMap by clicking the red toolbox icon.

Since you have found that the dataset you downloaded already has a projection defined (GCS - NAD83), you do not need to define a projection, but can simply project it into another projection of your choosing.

Side Note: If you ever come across a dataset that has no projection defined, it is likely that it may be in a GCS projection. Try defining it as GCS, and lay it over some data of a known projection in the same geographic region and see how it lines up. Datasets with no projection defined might also be in a State Plane projection, especially if they are state level or smaller data, so try that next.

In ArcToolbox, go to Data Management Tools >> Projections >> Raster and look at the available tools. There should be two sets of projection tools, one for features (vector data - points, lines, and polygons) and one for raster data, like elevation grids. There are other tools for projection adjustment and transformation, etc.

Since we want to re-project a grid with a projection already defined, select "Project Raster " and double-click on it to open the wizard.

In the first window, select "Project my data to a specified coordinate system" and click Next to continue.

In the next window, use the browse button to find and select your NED dataset. Be sure to change the Output Cell size to 30 (meters) once you have set the coordinate system (below).

Click Next and click Select on the following window:

This gives you the opportunity to browse through Geographic (latitude and longitude) and projected (State Plane, UTM) coordinate systems.

Select the State Plane projection system. The description notes that "State Plane" is not strictly a projection itself, but rather a code system dividing the US into regions (mostly by state), each of which has it's own projection, optimized to each region. Click Next to continue and navigate to Projected Coordinate Systems, State Plane, and NAD 1983 and then choose the state that you retrieved data for.

When all is ready, click OK to run the wizard.

Wait for the "Processing..." window to close. Your data has been projected and a new dataset created. It is automatically added to your map and it won't be evident that it has been, unless you look at the Table of Contents. That is because the new layer, the projected DEM, has projection information stored within it and the program projected the data to the projection of your original data (Geographic).

Changing the projection of a map

The projection is changed by changing the data frame properties. The data frame is the map area of ArcMap. To change this, click on the View menu and then click on Data Frame Properties. You should see a form like this one:

You can change your projection to any of the available projections, or to the projection of any of the layers in your map. In this case, I navigated to the layers list, clicked on my new projected layer, and then chose its projection. Click ok. You should see the map change somewhat. If you can't see your data, right click on the name of the new, projected grid and then click on Zoom to Layer.

 

created by Christiaan Adams
last updated 9/26/05 - Daniel Sheehan