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I want to add a scale into my map but it is not accurate. I've set View > Date Frame Properties to Kilometres and I set the scale to Kilometres but I think it is only showing Decimal Degrees.

How do I create a scale in ArcGIS that accurately reflects on-the-ground distances?

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Change your data frame spatial reference to a projected spatial reference. ArcGis has the capability to project-on-the-fly, so your data may be in DD but can be shown in projected coordinates, this will allow you to use a scale bar in kilometres. – Michael Miles-Stimson Aug 21 '14 at 4:43
I've set both units to Kilometres in Date frame properties > General. Where do I change it to projected spatial reference? – Tim Forssman Aug 21 '14 at 4:48
@Michael Good comment, however, it would make a great answer;) – Aaron Aug 21 '14 at 4:50
up vote 4 down vote accepted

In order to use a scale bar with real measurements you must use a projected coordinate system, for example: WGS84 UTM (Zone XX north or south), Albers Equal Area (for large areas) or Lamberts Conformal Conic projection.

Select View::Data Frame Properties or right click on the data frame iconenter image description here in the table of contents and go to the properties. In the view there is a tab Coordinate System which will allow you to change the spatial reference of the map. If it is greyed out there is a problem with one of your layers or you are currently editing - in either case you cannot change the spatial reference of the map. enter image description here

Of course, the correct coordinate system for the area you are covering depends entirely on where it is in the world, what datum your data currently is in (your geographic coordinate system) and how much area you need to cover - sometimes it can be tricky to get it right and worthy of a GIS.SE question in itself.

If you are only covering a small area (less than a whole state or country) then try UTM; in the reference systems folder of you Esri install location there is a shapefile called UTM which will give you the zone number to use, then select either north or south depending on whether you are above or below the equator (if you are on it doesn't matter) provided you aren't outside of the arctic circles... Then find in the Projected coordinate systems/UTM/WGS84/your hemisphere/ WGS 1984 UTM Zone your zone.

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Great, thanks. I have tried it but now it says that there is a conflict between some layer's projections and the new projection (WGS_84_world) and after I transform it they do not show up together any more. The scale still doesn't work (the units are way too small). – Tim Forssman Aug 21 '14 at 5:17
It should be WGS84 UTM. You may need a transformation for your data (there's a drop-down list). What datum is your data currently in? – Michael Miles-Stimson Aug 21 '14 at 5:24
There are three datums. GCS_Cape, GCS_WGS_84 and Lat Long for MAPINFO type 0 Datum. It asks to convert all of these in the transformations tab. The co-ords are around 22 S, should I select WGS 84 UTM zone 22S? – Tim Forssman Aug 21 '14 at 5:34
Refer the shapefile UTM.shp, the zones are 6 degrees wide. GCS_Cape puts you in Africa somewhere which should be around 24 south. Use the suggested transformations except for MAPINFO type 0, you will need to find out what it actually is and not just the default geographic coordinate system, for that you may need to read the metadata statement or contact the vendor. – Michael Miles-Stimson Aug 21 '14 at 5:39
Thanks so much. I will give it a try – Tim Forssman Aug 21 '14 at 5:46

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