I have been sent a layer which has a progressive error in longitude away from the meridian (where it is correct). The CRS claims to be GCS ED50 and the latitudes all appear to be correct. I’m wondering if a truncation or rounding to insufficient significant figures of one of the CRS parameters in the source software might be to blame but wondered if anyone had seen such an error before and could suggest things for me to check?
I’m looking at the North Sea and the meridian is 0 degrees in case that helps.
Are you talking about a graticule (lines of latitude and longitude) or a graticule layer? And it's being projected on-the-fly to which projected coordinate system? We did have a case in ArcMap that a graticule doesn't use enough points to build the line. You could use the ArcMap Advanced Settings Utility (in the installation folder, Utilities) to change the densification setting.
Otherwise, depending on how wide an area you're displaying, there could be a projection algorithm issue, particularly if that data wasn't built in ArcGIS.
It's not a graticule but, the polygons are made up of lines which only ever follow the lines of lat / long so i suppose you could think of it as an irregular graticule?? Each polygon is only a maximum of ~12 km wide (West East) and ~20 km high (North South) so I don't think it's a density issue.
My suspicion is that a projection algorithm problem exists somewhere back up the chain, probably not in ArcGIS. I'm trying to get my head around which CRS parameter would need to be 'off' (or not specified to the correct level of precision) to result in the magnitude of the longitude being a small percentage off as the distance from the meridian increases.