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| How do I display the scale bar in miles or kilometers when the XY coordinates are in degrees of latitude and longitude? |
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 First convert the map X Scale and Y Scale to Cartesian coordinates (i.e. miles, feet, kilometers, meters), because one degree of latitude does not cover the same number of miles as one degree of longitude. For an approximate conversion from latitude and longitude degrees to miles or kilometers, use the following formulas for a spherical projection:
1 degree latitude = 110.6 km
= 68.703 mi
1 degree longitude =
= 111.3km * cos(latitude)
= 69.172mi * cos(latitude)
To set the scale bar of a map, use the formulas to convert latitude and longitude to the desired coordinates. For the map of Nevada USA included with Surfer (nv.gsb) the center of the map is approximately (-117, 39). Using the formulas:
1 degree latitude = 68.703 miles
1 degree longitude =
= 69.172 * cos(39)
= 53.757 miles
Use this excel spreadsheet as a convenient conversion calculator.
To set the X and Y to the same scale in miles, select the map and choose the Map | Scale menu. Disable the Proportional XY Scaling checkbox in the lower left corner of the Scale dialog box. If the X scale is set to 1.0 in = 1 Map units (degrees longitude), calculate the ratio of X to Y as 53.757mi / 68.703mi = 0.78245. Specify the Y scale as 1.0 in = 0.78245 Map Units.
Disable Proportional XY Scaling, and specify the X and Y Scale.
The default scale bar is created in degrees with a Cycle Spacing = 1 and Label Increment = 1. To change the scale bar to 50 mile increments, change the Cycle Spacing to 50mi / 53.757mi per degree or 0.93 (degrees) and the Label Increment to 50 (miles).
Set the Cycle Spacing in decimal degrees
for the desired Label Increment.
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