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Last updated
6/19/02

Reprojecting Geospatial Data: Procedures


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Using ArcInfo

PROJECT

Most reprojections can be performed at the Arc: prompt with the command PROJECT. The syntax for this command is:

PROJECT <COVER | FILE> <input> <output> {projection_file}
PROJECT <GRID> <input> <output> {projection_file} {NEAREST | BILINEAR | CUBIC} {out_cellsize} {x_register} {y_register}

After typing in the initial command, Arc will require several subcommands that define the parameters of the projections. If you are starting with a data set that does not have a defined projection, then the first thing you type in is input. This tells Arc that you are about to define the initial projection. After entering the parameters for the input projection, type in the subcommand parameters, hit [Enter] and then type in output. Now Arc is ready to receive the parameters for the new projection. Once those parameters have been entered, type in parameters, hit [Enter] and then type in end. Arc will now reproject your original data set to the new data set with the name you specified in the initial command. Please look up the PROJECT command in the ArcDocs to view all of the different subcommands.

Below are examples of three different ways to project a data set.

  1. Search for "projections,supported" in the ArcDocs index. This topic will show you the all of the supported projections and their parameters. If you click on "Related Topics" while on this page, you can view the supported datums and spheroids.

  2. If you have a single data set to project, you can just type in everything at the prompts as follows:

    • Arc: project grid dem10m dem10m_utm # # 1000
    • Project: input
    • Project: projection lambert_azimuth
    • Project: units meters
    • Project: datum none
    • Project: parameters
    • Radius of sphere of reference: 6370997
    • Longitude of center of projection: -100 0 0
    • Latitude of center of projection: 45 0 0
    • False easting: ~
    • False northing: ~
    • Project: output
    • Project: projection utm
    • Project: datum nad83
    • Project: units meters
    • Project: parameters
    • Project: end
    • Arc:

    If your original data set (dem10m) already had a defined projection, then you would omit the commands from input to the last ~.

  3. You can use a command like this if you have a standard projection file to apply to several data sets:

    • Arc: project grid dem10m dem10m_utm laztoutm

  4. You can also use an AML file (very handy if you have several data sets to reproject because you can program in loops to process through all of the coverages/grids/files).

PROJECTGRID

This command can be used at the Grid: prompt and it works faster than using PROJECT at the ARC prompt because it reprojects the entire grid with a best fit polynomial instead of cell by cell. Nearest neighbor analysis is the default polynomial used to reproject the grid. The syntax for this command is:

PROJECTGRID (<grid>, {projection_file}, {NEAREST | BILINEAR | CUBIC}, {NINE | SEVENTEEN | FULL}, {out_cellsize}, {x_register, y_register})

As with the PROJECT command, you can use a projection file to specify the input and output projections. You can also use the AML file, or type in the commands at the prompts.

  1. Here is an example of typing in the commands at the prompts:

    • Arc: grid
    • Grid: dem10m_utm = projectgrid (dem10m, #, #, #, 1000)
    • Project: input
    • Project: projection lambert_azimuth
    • Project: units meters
    • Project: datum none
    • Project: parameters
    • Radius of sphere of reference: 6370997
    • Longitude of center of projection: -100 0 0
    • Latitude of center of projection: 45 0 0
    • False easting: ~
    • False northing: ~
    • Project: output
    • Project: projection utm
    • Project: datum nad83
    • Project: units meters
    • Project: parameters
    • Projectgrid: end
    • Grid:

    If your original data set (dem10m) already had a defined projection, then you would omit the commands from input to the last ~.

  2. You can use a command like this if you have a standard projection file to apply to several data sets:

    • Arc: grid
    • Grid: dem10m_utm = projectgrid (dem10m, laztoutm)

  3. You can also use an AML file (very handy if you have several data sets to reproject because you can program in loops to process through all of the grids).

PROJECTDEFINE

This command allows you to simply define the projection of a coverage, grid, tin, or file at the Arc: prompt. This can be done on data sets with no existing definition, or it can be used to change the projection that was previously defined. The syntax for this command is:

PROJECTDEFINE <COVER | GRID | FILE | TIN> <target>

Some sample commands are:

  • Arc: projectdefine cover rivers
  • Arc: projectdefine grid dem10m
  • Arc: projectdefine tin pikepeak
  • Arc: projectdefine file birdsite.dbf

This command tells ARC that you wish to define the projection for a certain coverage/grid/tin/file. After you enter this command, ARC will require several subcommands. The subcommands are the same as those specified for the PROJECT command except that you do not need to specify if the parameters are for the input or output projection since you are not reprojecting anything (see above for details).

An example of an entire PROJECTDEFINE process is:

  • Arc: projectdefine grid dem10m
  • Project: projection lambert_azimuth
  • Project: datum none
  • Project: units meters
  • Project: parameters
  • Radius of sphere of reference: 6370997
  • Longitude of center of projection: -100 0 0
  • Latitude of center of projection: 45 0 0
  • False easting: ~
  • False northing: ~
  • Arc:

This can also be done by simply running an AML file (very handy if you have several data sets to define projections for).





Using ArcView

To reproject a shapefile in ArcView 3.x, first make sure that the Projection Utility extension is added. Click on File and then Extensions. Make sure that there is a check next to Projection Utility Wizard and click OK.

Make the shapefile of interest the active theme. Click on File and then ArcView Projection Utility. You will be presented with the first screen of the projection utility. This screen allows you to view the current projection of the shapefile. You can use the Browse button to choose a different shapefile.

Click on Next to choose the projection, datum, units, parameters, and ellipsoid of the new shapefile. The only required input is the coordinate system type and the units, but the Show Advanced Options checkbox will allow you to change the other items. Searching for "Projection Engine" in the ArcView help documents will allow you to view the supported projections and datums for different areas of the world if you are unsure what to choose.

Once you have decided what the new projection properties will be, you need to click on Next to choose the location and name of the new shapefile. Click on Next again and you will see the verification screen for the projection properties of the new shapefile. Once you click on Finish, the wizard will close and you will have the option of adding the new shapefile to the view.

These are the four screens in the Projection Utility in ArcView 3.x.
  1. Projection Utility: Screen 1

  2. Projection Utility: Screen 2

  3. Projection Utility: Screen 3

  4. Projection Utility: Screen 4





Using ArcGIS

Defining Coordinate Systems for Data Sets

If you have a data source without a defined projection, you must first use ArcMap, ArcCatalog, ArcToolBox, or a text editor to define its projection and coordinate system. Without this step, you will be unable to perform tasks in the GeoProcessing Wizard despite defining a projection for the data frame in ArcMap. This is a quote from the ArcGIS help documents that explains how to define the coordinate system for different data sources:

ArcMap expects coordinate system information to be stored with the data source. For a layer in a geodatabase, this information is part of the layerís metadata. For coverages, shapefiles, and rasters, itís stored on disk in a separate file named after the data source but with a .prj file extension (for example, streets.prj). These files are optional files; thus you may still need to define the coordinate system for one of these data sources. You can create a .prj file using ArcCatalogTM.

Here is an article describing how to use ArcToolBox to define a coordinate system for a shapefile. This article describes how to define projections for geodatabases, shapefiles, coverages, and CAD data.

Reprojecting Data Sets Without Altering the Data (Data Frame Properties)

To define the projection of a data frame, right click on the data frame name and choose Properties. By default, the data frame is called "Layers." This will bring up a screen like Figure 5. Click on the Coordinate System tab. You can navigate through the folders in the bottom half of the screen until you find the desired projection/coordinate system. Once you click on Apply or OK, ArcMap will reproject all of the data sets in your data frame without altering the data itself.

If one of your data sets does not reproject, make sure that it has a defined coordinate system (see above). You may also need to use the Transformations button to define how ArcMap should transform data sets with different geographic/datum coordinate systems (Fig. 1). This will have to be specified if there is more than one possibility for ArcMap to choose from. You can read more about this topic here.

The "Data Frame Properties" screen, "Coordinate System" tab.

Fig. 1. The "Data Frame Properties" screen, "Coordinate System" tab.

Using the GeoProcessing Wizard

Before using the GeoProcessing Wizard (Fig. 2), you need to define the coordinate system for the data frame that includes the data sets you are going to manipulate (see above). Then you simply use the GeoProcessing Wizard (found under the Tools menu) to perform whatever task you had in mind (dissolve, merge, clip, union, or intersect). The wizard will automatically reproject the new data set to the projection/coordinate system defined in the Data Set Properties, no matter what the projections of the original data sets are.

The GeoProcessing Wizard

Fig. 2. The GeoProcessing Wizard.

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