Projecting Latitude and longitude
I have data in an Excel file with latitude and longitude references in decimal degrees, and when I project the data as XY data in ArcGIS it isn't projected correctly. It will also say coordinate system unknown. When I project the data just by itself, it projects where it is supposed to be, for example, 44deg and -75deg. When I add an additional layer of pre-made data (state map) projected in NAD83 UTM 18N, my data points are no longer projected correctly. Is there anyway to references the points in latitude and longitude or convert the latitude and longitude to a different reference system within ArcGIS?
Mapping Center Answer:
The first thing you should do is look at the desktop help to make sure you added your XY data correctly. There are step-by-step instructions on Adding x,y coordinate data as a layer.
It sounds like you might not have defined the coordinate system your data should be in and therefore are getting the coordinate system unknown problem. When you add XY data, you can't leave the coordinate system set to unknown. If you don't set it, your lat/long coordinates are displayed relative to your data frame's coordinate system.
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