General Description of Projections
Excerpt of ‘Setting the map projection’ in ArcView Help:
Map projections allow areas on the surface of the Earth (a spheroid) to be represented on a map (a flat surface). The type of map projection used on a map affects both its aesthetic and geometric qualities, although for most general purpose ArcView applications these impacts are not large. See Map projections for more about projections and a list of the projection and coordinate systems available in ArcView.
Projections in ArcView
Excerpt of ‘Setting the map projection’ in ArcView Help:
A view's map projection can only be set if the map units of the spatial data it contains (or will contain) are decimal degrees (i.e., degrees of longitude-latitude expressed as a decimal rather than in degrees, minutes and seconds). This is because data in decimal degrees is in a spherical coordinate system and so is, by definition, unprojected. This data can therefore be drawn in any projection in ArcView. The map projection used by a view is set in the View Properties dialog box.
To set a view's map projection
- From the View menu choose Properties. The View Properties dialog box will be displayed.
- Press the Projection button. ArcView displays the Projection properties dialog box.
- Choose the projection you want to use. If desired, you can also set custom projection parameters.
- Press OK on the Projection Properties dialog box. ArcView sets the Map Units to "projected meters". This indicates that the view is projected, and that the map units are meters. If the map units are changed from projected meters to any other units, for example, decimal degrees, then the projection is removed. If you choose "projected meters" without setting a projection, ArcView sets the projection to Robinson.
- Press OK on the View Properties dialog box. ArcView redraws the view using the projection you chose.
If your data is in decimal degrees and you set the Map units to "decimal degrees" but you choose not to set a projection, ArcView will draw the view by simply treating the longitude-latitude coordinates as unprojected spherical coordinates. This produces a map that looks the same as one that has been projected with the Plate Carree map projection, although its geometric characteristics are not the same.
The Projectr Project
The ‘Projectr’ project is an ArcView project which contains several scripts and a custom button designed to allow geographic data to be projected from one coordinate system to another using the projection features of ArcView. It was developed by Tom Schwartzman [ESRI-Boston] and is available on the internet at ftp.esri.com/pub/arcview2/projectr.apr . Note that this project has been enhanced in the past and may be enhanced in the future. It is to be reissued for ArcView v3.0 as an ‘extension’.
This project should be obtained and opened or imported. This will provide you with a custom button on the View GUI which will project the active themes to a specified projection. This button appear as a north arrow to the extreme right of the button bar. You may -click on this button for the following help text:
To use the projection tool you must know the map units and projection of the datasets being projected. You will be prompted for the output units and projection.
- Add some themes to the view.
- Set the map units appropriately in the View Properties window.
- Make the theme(s) you wish to project active.
- Press this button. You will be prompted for certain information which may include the input projection, the output units and the output projection.
- You will be asked if you want to add the projected theme(s) to a view.
- You will be asked for output shapefile names for each theme to be projected
This script is unsupported by ESRI. If you have problems with it, feel free to email the author (firstname.lastname@example.org), but I make no promises about answering (although I will try).
Creating a Custom Projection
A custom projection can be defined by selection the ‘Custom’ button and typing in the appropriate parameters. Note that a custom ellipsoid cannot be defined in this dialog. This also requires the user to record and type all parameters.
A custom projection can also be defined in a user custom projection file. This file is read in by ArcView each time the Projection Properties dialog is opened thereby preventing the need to type in the parameters each time. This method also allows for a custom ellipsoid which is a requirement for County coordinates, for example.
Name: "Custom Projections"
Description: "Dakota County - 83"
The default.prj file is best created using a script such as ‘MkDakPrj.ave’ as shown in the following example.
‘ MkDakPrj.ave - Make projection file for Dakota County projection.
‘ This script creates a projection file called default.prj in the user’s home directory.
‘ This file is read in when the projection dialog is activated. This custom projection
‘ will appear in the category ‘Custom Projections’ in the projection properties dialog.
pj = Lambert.Make(Rect.Make(500000@100000,650000@250000))
pj.SetDescription("Dakota County - 83")
sp = pj.GetEllipsoid
' Create a CoordSys object and get its list of
c = coordsys.make
projections = c.getprojections
' Add the new projections to the CoordSys projections
' Create a default.prj file with ODB
' and add the CoordSys object
defprj = odb.make("$HOME/default.prj".asfilename)
After this script is executed, the custom projection will appear in the Projection Properties dialog (in the view properties and when executing the projector script.
This document is a collection of information found in other places, specifically the ArcView Help system and the ESRI web site. This is not intended to represent original information or infringe on copyright laws but merely as a summary of those sources. Please refer to the sources noted for additional information.