On-the-fly Reprojection

Tuesday, February 21, 2006 at 08:54:10

I had some x-y coordinates in geographic lat/long that I wanted to display with some Ordnance Survey 1:50,000 raster data (OS National Grid) as a backdrop. This point data was supplied as a comma separated text file (CSV) which you can open up (amongst other programs) in a text editor or Excel. I needed to load it in to ArcMap which unfortunately doesn’t understand CSV files for tables (at least not obviously). I used Excel to save it as a dbase IV file; it didn’t work properly and, after a bit of investigation, I found that Excel did not save the dbase file as floating point data. This time I went to Access, loaded the CSV file in to there, saved it as an Access database and then loaded that table in to Armap. Fine. You can now use the “Add X-Y Data” from the Tools menu to turn the table (with x-y coordinates) in to plottable data. That worked fine and, after saving it as a shapefile, I overlaid it on to the OS backdrop. Except all the x-y data were ~300m out. Aghhhhh! I loaded exactly the same data set in to ERDAS Imagine and, well, it worked perfectly!!! Clearly the on-the-fly reproject of the lat/long coordinates in to OSGB works in Imagine and doesn’t in ArcMap. This is very frustrating and required the conversion of the shapefile to a coverage (in order to reproject it), reprojection in to OSGB before finally reloading it. This solved the problem, but why-oh-why can’t we have proper quality control on crucial low-level operations such as this?

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