When Cartographica displays a scale, that scale is computed in units that are appropriate for the entire map being displayed. This means that unit choices will be limited based on whether the current projection (a type of coordinate reference system) is displaying linear or angular units.
If the map is displaying angular units (degrees), then you will need to choose Map > Project Map... to change the map display to a projection that results in a linear CRS. Once you have done that, the scale will automatically change to match the default for that projection and you will be able to switch between linear units.
The reason for this requirement is that the relationship between degrees and linear units is never constant across a vertical area, and therefore would be inaccurate for most of the map.
The opposite is also true. If you want to display in degrees, you will need to use an angular coordinate reference system (such as WGS84).
Although this is by no means a comprehensive list, here are some suggestions on finding appropriate linear projections:
- If your Map covers most of the US, we would suggest using a projection such as Lambert Conforming (USA) or Albers Equal-area (USA).
- If it covers area within one US state, we suggest using one of the State Plane projections for that state.
- If it covers the UK, we suggest using OSGB (Ordnance Survey National Grid).
- For other ares of the world, the UTM Zones are appropriate if you do not easily find a local coordinate system. For example, Botswana would be UTM Zone 35 South.
- And for areas that cross continents, each of the world-wide projections has some level of validity, but they will all be inaccurate for some areas, due to the aforementioned differences between angular and linear measurements.
Projecting the Map vs. Reprojecting the data
We generally suggest using the Project Map command instead of reprojecting the individual layers because this allows the individual layers to remain in their original format and with their original values. Reprojection changes the data in the layers, possibly causing a reduction in accuracy if the coordinates are reprojected multiple times. Projecting the Map, on the other hand, allows Cartographica to reproject the data as necessary to match the output CRS, but leaves the original data intact.