Wrong scale displayed when using Mars datum

danielek

I noted for some time, though I can't remember when, that the scale displayed in GM 26 with a Mars datum is wrong. See the attached screenshot. The correct distance is in the data information on the lower margin of the window. This wrong scale is shown for the default Mars datum, as well as other Mars datums I defined manually with slightly different axes.

It is not a simple Earth to Mars scaling issue. The impact basin diameter from the scale bar would be 600 km, instead of 200 km in reality; a missing Earth to Mars scaling would give 400 km, not 600 km.

I did not check with GM 27, which has an distinct issue with extraterrestrial projections (from an earlier conversation).

Thanks :)

daniel

bmg_mike

Hi Daniel,

Is the output from the Measure Tool the value that you would expect?

The scale bar is not going to be accurate in an east-west direction with an unprojected system like Geographic because the scale in the north-south direction is significantly different than the scale in the east-west direction. At a latitude of about 50 degrees it's off by about 35%.

If you change to a projected system appropriate for the area, like perhaps a UTM, then the scale bar should be much more accurate in both directions.

The Measure Tool output will give distance across the ellipsoid (if you have Great Circle selected for distance measurement), so it should be accurate regardless of the selected map projection.

Thanks,

Mike

Global Mapper Guru

danielek

Hi Mike,

>>Is the output from the Measure Tool the value that you would expect?

yes

On other planets we unfrequently use local projection systems, but I tried the polar stereographic and centred it at the middle of the window view.

The Measure tool gives the expected results.

The discrepancy with the scale bar seems much smaller. A line that would be 100 km according to the scale bar is in fact 91-93 km long along a longitude line, depending on where the line is moved within the window, and like 116 km following a latitude line. This looks consistent with a scale bar measurement done from the projection parameters, perhaps within the window but taken obliquely of by doing an average based on the displayed lat and lon range (?)

The equirectangular or geographic "projection" is a standard on other solar system bodies below 65° or 70°lat, and it is by far the easiest way to map on irregular objects such as asteroids. In such cases, having the option of a scale bar based on some relevant geoid (which for planets is usually an ellipsoid) -- and even two scale bars, one vertical and one horizontal, would be terrific!

bmg_mike

Hi Daniel,

The scale bar should always be based on the ellipsoid of the currently selected datum, so it should obey the planetary ellipsoids. For a handful of highly distorted projected systems (like Equirectangular and Mercator), it will actually use a pixel-sized square at the center of the display to get a representative meters-per-pixel value for the scale bar. For other systems, it will use the meters-per-pixel along the north-south direction as that tends to be much less distorted.

It's actually impossible to have a single (or even multiple) scale bars that are entirely accurate. Imagine a display in Geographic that covers the entire northern hemisphere of some planetary body. The number of meters per pixel along the east-west axis will be significantly different at the bottom of the display (say around the equator) than the top. At 60 degrees north, each horizontal pixel is half as many meters across as the same pixel at 0 degrees north. It goes to 0 meters per pixel at the poles.

In a local projected system (say a Transverse Mercator centered on some smaller area), the distortion is small enough that it doesn't matter much, but in more distorted global systems it's not a solvable problem.

Mike

Global Mapper Guru