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Accuracy of GeoSpatial PDFs

Forum Index > General Acrobat Topics > Accuracy of GeoSpatial PDFs
2009-08-07 08:02:53
NicolaOBrien
Registered: Jul 22 2009
Posts: 2
Answered

Hi,
I'm creating georeferenced PDFs using ESRI ArcMap, and I include Gridlines in my map. However, when I open it in Adobe Reader or Acrobat, and use Find a Location function, the mark can be a large distance away from the grid lines, and when I zoom in the grid line can still be a large distance away. Depending on what type of map, it can be over 1 second away, or somtimes over 35 metres. I'm exporting maps created at 1:50,000 (at 200 dpi) so this error is too large for comfort. Does anyone know why this happens? I've tried exporting maps in WGS84 Geographic, and also in Projected co-ordinate systems, e.g. UTM 29N (WGS84), but still the same error occurs. Has anyone else had this problem? Have Adobe written anything about the accuracy of their GeoSpatial PDFs? I use both Acrobat and Reader on Windows.
Thanks
Nicola

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2009-08-08 01:58:10
#1
UVSAR
Expert
Registered: Oct 29 2008
Posts: 1357
The accuracy of the geospatial tools in Reader/Acrobat is mathematically good to the last decimal place (the calculations are floating-point), but in reality is never that "accurate", mostly because of the resolution of the screen at current zoom level (which decides how precise the XY mouse data is) and the precision of the corner pins. The georegistration process in ArcMap outputs the PDF as normal, then embeds the real-world coordinates of the four corners of the map, the projection/datum and the relationship between the map corners to their location on the document page (in Postscript points). From these four "corner pins" all the XY values are calculated dynamically, so if ArcMap rounds off any of those values, the geomeasure values will be offset and/or stretched. Acrobat has no way of knowing if the corner pins are out, so it can't display an "accurate to xxxm" warning on the geomeasure tools - it shows you as many decimals as it can fit on the screen and leaves you to work out how many to believe.

Taking the PDF apart again to determine the georegistration info is a fiddly process (can't do it in Acrobat, we have to decompile the PDF into raw data), so posting us an example is pointless - however I'm sure it's a problem within ArcMap and not within Reader/Acrobat, so advise you contact ESRI's support desk or post a query in their forums. geoPDF export is a new feature for ArcMap and people are still finding bugs with it.
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2009-08-10 05:07:37
#2
NicolaOBrien
Registered: Jul 22 2009
Posts: 2
Thanks for this, it does make sense, especially about the corner values being rounded off when the PDF is created. On some of the maps the error is only about 10m, which could easily be a rounding off error, but when it is combined with everything else makes for the larger errors seen on other maps.
I'll try the ESRI forum too.
Thanks
Nicola
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