measure distance on data with undefined coordinate system

[carinaluo]

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.

[HardolphWasteneys]

Yes.
The distance between any two points will be correctly calculated using the tool when the data is in a GCS and the Data Frame as well.  It is just measuring the arc length on the earth surface.

However, if you try to calculate any geometric properties of the features such as length (of a curving polyline) or area in a non-projected Data Frame you will find the functions disabled and get the message in the clip below that all calculations are done using "planimetric algorithms". 

[ATTACH=CONFIG]14667[/ATTACH]

You can check the distance aspect further by drawing a circle in a GCS Data Frame in a file with either a projected or geographic coordinate system (i.e. not using the graphic tool)and see the measured difference between the N-S and E-W diameters using the measure tool, the ratio of which increases at higher latitudes.  To see the GCS circle change shape in a Projected Data Frame use the generalize tool on it to make it into a series of points (offset .001 or something; or it will just recalculate as a curve into a new circle).  Then shut off editing and change the coordinate system to a projected one appropriate for the range of the data and observe N-S stretching and graphically correct diameter measurements.

I'm not quite sure what you mean by: "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". , but you can project the GCS data into any Projected CS you want for your layout, otherwise it will look flattened N-S.

hope that explains it well enough,

Hardolph