GIS Cookbook: Recipe - Reprojecting Data  
Keywords: Projections, datums, coordinates, NAD, conic, match, conformal, planar, Robinson, geographic, azimuthal, cylindrical, longitude, latitude, Mercator, scale, skews, rotates, and shifts coverage, transform, geodata, spheroid
Category: Projections
Software: ArcView 3.2

Problem: The features on my map don't line up. Data occupies the same space in reality but is not displayed this way onscreen, how do I remedy this? ArcView 3.2

Description: Data from a spherical surface (Earth) can be displayed and distorted in numerous ways when placed on a flat surface, therefore the different ways of placing this data on the page are called projections. Geographic projections are different mathematical techniques to convert and portray
features from a spherical surface (Earth) onto a flat surface; each map projection has certain strengths and weaknesses in terms of accurately representing shape, area, distance, and direction. Projections must be altered to make data line up and to present it in the most geographically effective manner.

Scenario:



buffer

Methodology:
The problem, the data doesn't line up.


1) Identify what projection your data is currently in if you have not already done so. To identify the current projection, go to View->Properties.


2) Make sure your map units are set within View->Properties. If you get an error while setting properties, See Pitfall 1.







What Projection am I in




3) Open File->Extensions. Engage (i.e. check on) Projector. If you do not have Projector follow the instructions after the next screenshot.







Here's how you can get Projector, a free and easy-to-use script, if you do not already have it:

    1) Go to http://www.arcscripts.esri.com, then type "projector" into the search line and hit the search button.

    2) Select the ArcScript projector (which is authored by ESRI), click on it and select download.

    3) Choose accept and download the file into your Ext32 folder, which itself is within your Av_gis 3.2 folder.

    4) Open the Ext32 folder and unzip your downloaded file into it. You will notice one of the results is a file with the ending .avx. This is your file extension.

    5) If you have ArcView open, close it and then reopen it.

    6) Choose File Extensions and click on the Projector extension.







4) Close the extensions window and you will notice that a new icon appears on your ArcTools bar. If the icon does not appear, go to See Pitfall 2.






5) To make the theme you want to reproject active, select the key for that theme and click the Projector button

6) You will then be guided by a wizard, which will help you choose your units (usually defaulting to meters), then choose which projection family and type you want or need to make your data match up properly. If it is unclear how your data is currently projected, you may want to select a standard projection for every theme, using the Projector tool for each. Finally, you will add the newly projected data to whatever view you choose, then name the theme and store it in the appropriate directory.











7) The result is that now all of the data is projected in the same manner (as shown below).




Pitfalls:
Links:
ESRI - Hardware and software help, including online courses and tutorials
NCGIA - An excellent resource for any social scientist using spatial ideas
The Geography Network - A place to go
ArcScripts page - Download projector from here
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Authored by: Benjamin N. Sprague Modified: 9/10/03


Copyright © 2002-2014 by Regents of University of California, Santa Barbara
Cookbook: Ben Sprague, Ethan Sundilson, Carlin Wong, Sam Ying