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GCS GDA94 Coordinate System Distortion in Scale

Question asked by geonetadmin on Jun 20, 2010
Latest reply on Jun 21, 2010 by geonetadmin
Original User: samknightnz

I have been working with a dataset that contains polygons for features all around Australia, so the geographic coordinate system GDA94 has been used for the shapefiles and data frame. This is required so that lat/long can be used and avoids splitting the data up into UTM zones.

The problem I have found is that there is distortion in the horizontal dimensions of these features, as shown in the PDF example attached. The numbers displayed are measurements taken using the ruler tool in the data view, but when these are compared to the scalebar it becomes apparent that the feature is not displayed at the correct size relative to the measured lengths. I have added the red lines to show that on one side, the measured distance is 52.5km but the scalebar shows it as being over 55km- quite a significant difference! If a scale ruler is used on the printed map it confirms that the scale bar is accurate but the widths of polygons are wrong (although heights appear to be correct).

To double-check I have imported the polygons into another GIS package and found that the widths displayed and printed correctly, so this seems to be something specific to ArcGIS and not to the data. The shapefile for the polygon used in this example is also attached.

Has anyone encountered this before and are there any solutions to correct the transformation in ArcGIS?

Thanks in advance

Sam

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