Projecting Data

Introduction

Map projections are attempts to portray the surface of the Earth or a portion of the Earth on a flat surface. A geodetic datum is a geometric model describing the size and shape of the Earth and the origin of the coordinate systems used to map the Earth. There are a number of map projections and datums, each of which is useful in dealing with particular scales of data or different regions of the world. As a result, geographic information system (GIS) databases often occur in a variety of map projections and geodetic datums.

All of the spatial data provided in the Northern Puget Sound Ecological Characterization (NPSEC) are in a map projection of Universal Transverse Mercator (UTM) Zone 10, using the North American Datum of 1927 (NAD27). A number of other map projections are sometimes used with spatial data. For example, much spatial data exists in either State Plane or Geographic projections. It is also not uncommon to encounter data based on a more recent datum, such as the North American Datum of 1983 (NAD83). For these reasons, it may be necessary to change projections or shift datums when working with data from different sources in order to make the data layers align properly.

This tutorial will describe how to to change the projection of a file from State Plane to Universal Transverse Mercator using the spatial data that were downloaded in the previous exercise (Retrieving Internet Data).

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Viewing Spatial Reference Information

Recall that the Interchange (.e00) file downloaded in the previous exercise (Retrieving Internet Data) was converted to a "coverage" using ArcToolbox®. The name of this coverage is "dairy_2002" and is located in the directory you specified during the previous exercise. Before using this coverage, make sure it is in the same projection and datum as the spatial data provided in the NPSEC GIS. To view the spatial reference information associated with the coverage you will need to look at the metadata. You can easily do this by opening the coverage in ArcCatalog® and clicking on the Metadata tab.

Start by opening ArcCatalog, and then locate and click on the dairy_2002 coverage. Now click the Metadata tab and then the Spatial tab within the Metadata window. You should now see an image very similar to the following:

As you can see, the projection for dairy_2002 is State Plane and the datum is the North American Datum of 1927. In order to accurately line up the dairy_2002 data layer with the NPSEC GIS data, the projections and datums need to be the same. Recall that all of the NPSEC GIS spatial data are in a map projection of Universal Transverse Mercator (UTM) Zone 10, using the North American Datum of 1927 (NAD27). Thus, the dairy_2002 spatial data and the NPSEC GIS spatial data are in the same datum, but are not in the same projection. The projection of dairy_2002 needs to be changed to UTM Zone 10.

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Exporting an ArcInfo® Coverage to an ESRI® Shapefile

In order to project your dairy_2002 coverage using ArcView® 8.x you must first convert it to another common spatial data format called an ESRI® shapefile. To do this, start by opening the ArcView application ArcMap® and then adding your dairy_2002 coverage. Once open, right-click on the dairy_2002 layer and choose the Data > Export Data option. This will allow you to save your coverage as a shapefile, which has a ".shp" file extension.

A new dialog should appear that allows you to export (convert) your coverage to a shapefile. Choose the default settings, but rename the new output shapefile dairy_2002.shp and then click OK.

If the conversion was successful, a new dialog will pop up asking you whether you would like to add the exported data to the map as a new layer. Click No and exit ArcMap.

You have successfully converted your coverage to a shapefile that can now be projected using ArcView's application ArcToolbox.

Projecting Data

Start by opening ArcToolbox. Expand the Data Management Tools list and then expand the Projections list. Now double-click on Project Wizard (shapefiles, geodatabases).

A new dialog should pop up asking you to select the data you want to project. Start by clicking the folder icon on the right side of the dialog box.

In the resulting pop-up dialog box, locate the dairy_2002.shp file that you created earlier, and then click Add.

You have now specified the data you wish to project so click Next.

A new dialog should appear asking you to specify a location for the projected data, as well as a new file name. Start by clicking the folder icon on the right side of the dialog box.

In the resulting pop-up dialog box, specify a directory in which to save the projected file. Name the file dairy_2002_utm.shp and click Save.

You have now specified a directory name and location for the projected data to reside. Click Next.

A new dialog should pop up asking you to select the coordinate system you want to assign to the data. Start by clicking the Select Coordinate System button near the bottom-right side of the dialog box.

In the resulting pop-up dialog box, click Import. This option will allow you to quickly set your coordinate system to that of an existing data layer. In this case, you will use the NPSEC Project Boundary data layer. If you do not already have a copy of the NPSEC Project Boundary layer, download it from the NPSEC Data Inventory.

After choosing the Import option, a new pop-up dialog appears. Browse your data and find the NPSEC Project Boundary layer (Boundary.shp). Choose this layer and click Add.

A new dialog should appear displaying the spatial reference properties you have chosen. Check to ensure the name of the coordinate system is correct. The spatial reference information should match that of the NPSEC Project Boundary layer: North American Datum of 1927 (NAD27) and Universal Transverse Mercator (UTM) Zone 10N. If everything is correct, click Apply and then OK.

Accept the default coordinate extent values in the following dialog box and click Next.

A new dialog should appear that provides a summary of your input throughout the Project Wizard process. Check the input. If it is correct, click Finish and exit ArcToolbox.

You have just projected the dairy_2002.shp shapefile to match that of the NPSEC GIS spatial data.

Remember: The name of the newly projected shapefile is dairy_2002_utm.shp.

The metadata should now reflect the changes you made to the dairy_2002.shp shapefile. To view the metadata, open ArcCatalog and locate the dairy_2002_utm.shp layer. Click the Metadata tab, and then click the Spatial tab within the Metadata window. You should now see an image very similar to the one below. Notice that the new projected coordinate system is NAD_1927_UTM_Zone_10N.

You can now combine the dairy_2002_utm.shp layer with any of the NPSEC GIS data layers.

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