measure distance on data with undefined coordinate system

Discussion created by geospatialanalyst on May 25, 2012
Latest reply on May 25, 2012 by Hardolph
I want to measure two points which are stored in a geographic coordinate system using the measure tool on the Tools toolbar in ArcGIS 10.

But as described in the ArcGIS Help document,  

"If your spatial data references locations with latitude and longitude, ArcMap draws the data by simply treating the latitude-longitude coordinates as planar x,y coordinates." "Degrees of latitude and longitude are not consistent units of measure for area, shape, distance, and direction."

It seems to me that the distance calculated on data in a geographic coordinate system would be incorrect. However, in ArcGIS 10, "When measuring in a data frame with a geographic coordinate system, the default measurement type is Geodesic," which is "the shortest line between any two points on the earth's surface on a spheroid".

Does it mean that I can use the measurement tool to measure distance on unprojected data anyway?

Since I don't know what the coordinate system of my data is, I cannot set up a good map scale on a layout as ArcMap doesn't know what units the coordinates are using.

Can someone please help me figure this out? Many thanks in advance.