Excel table to ArcGIS not plotting correctly

Question

Hoping this is in the right section. I have several shapefiles that I need to pull the attribute tables from, data cleanse in Excel and then load back up to ArcGIS as a single file. I have modified the attribute tables to create a decimal degrees longitude and latitude in British National Grid co ordinate system, same as all my features and maps. If I try and then upload the data back to ArcGIS, by adding the Excel file to the map and using the Display XY Data pop up, the data is drawn miles out on the map and all the data points are clustered together.

I have looked around various forums and can't work out what I'm missing, so any advice would be appreciated.

MelitaKennedy · Answer

Hi Andy,

You can leave it. There's always a slight performance hit due to reprojecting the data that depends on the amount of data that you're looking at. You could convert the data to a feature class and make a copy in BNG by projecting it.

Melita

AndyKeljarrett · Answer

Hi Melita,

That's exactly what they were doing. Fairly new to this so still wrapping my head round the co-ordinate systems. I have now imported the data as OSGB 1936 and this is plotting correctly. Possibly a stupid question but is there anything else I need to do? Just because this feature doesn't have a projected co-ordinate system attached, unlike anything else in my map.

Thanks
Andy

MelitaKennedy · Answer

As Jim says in his response, it's almost certainly an incorrectly defined coordinate system. You state that you added attribute fields with:decimal degrees longitude and latitude in British National Grid co ordinate system

That's two different coordinate systems--a geographic one using longitude and latitude (OSGB 1936)--or British National Grid--a projected one. My guess is that the Excel data does have latitude-longitude values, but that you're assigning British National Grid as the coordinate system. That would cause the points to bunch up as they're all very small if their coordinates are being treated as BNG. Are they far southwest of where they should be?

Melita