# Should I Reproject Data Which Has Been Captured Incorrectly?

I'm working on a project for a client which is migrating data from an Oracle database to an ESRI Geodatabase. When data for this project was initially captured over 15 years ago, the coordinate system used was either UTM Zone 56 South or GDA94 MGA Zone 56 because much of the data was for South East Queensland, Australia. However, as the company grew, they continued to add data further west and maintained the same coordinate system. The data now extends as far west as Darwin, Australia which is in Zone 52.

My instinct is to reproject the data to GDA94 Lat/Long. However, I'm not sure if this will make things worse or better.

I have two questions:

1. How much error is in the data which is 4 zones out (i.e. in metres/kilometres)?
2. Will I introduce more error by reprojecting the data to GDA94 Lat/Long? Or is this the right approach?
• In (1), are you asking about scale distortion in the UTM coordinates are about positional error? We can respond to the first sense--the scale distortion is 7.5% in all directions at Darwin--but it doesn't seem like you have provided any information to respond in the second sense. BTW, there's nothing more "incorrect" in what this client did that there is in Google using a Mercator projection to map parts of the earth further than 20 degrees away from the Equator! Mar 12, 2013 at 23:54
• @whuber, how did you calculate the 7.5% for scale distortion? What information is required to determine positional error?
– Fezter
Mar 13, 2013 at 0:31
• This is part of calculations of the Tissot Indicatrix. As far as positional error goes, you just compare the distance and bearing as computed in the projection to the distance and bearing as computed in geographic coordinates. But in a very real sense that's not "error" at all--it's metric distortion in the map and will automatically be corrected upon reprojection. To assess actual error in the positions, you need ground truthing for comparison. Mar 13, 2013 at 3:21
• What version of ArcGIS do you have access to? It will affect my answer. Mar 13, 2013 at 6:41