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The File Coordinate System window let's you choose when to load surfaces with their native coordinate system, and when to use the coordinate system currently in use.

File Coordinate System pop-up



COORDINATE SYSTEM CONVERSION

Files for different maps may use different units for the coordinates used to locate items on the map.  For instance, one map might use latitude and longitude, while another might use meters north and east of some reference point.  When a map is loaded which uses a system that differs from the system used by other maps on display, one of two things will happen: either the data in the map being loaded will be converted to match the system used by the other maps, or the other maps will be removed from the display.  You may choose when to convert the new data so as to retain the old maps.

Your choices are:

    Never.

Usually maps are best displayed with their native coordinate system.  This choice ensures that no conversion will take place, which also allows the data to load quicker.
    When map series is unchanged.
Most maps in any map series (i.e., large, intermediate, small) will share the same coordinate system as their neighbors.  But sometimes two neighbors may be on different sides of a boundary between two coordinate systems (e.g., two UTM zones).  This choice allows you to display two such neighbors.  This can be a good compromise because it allows you to span these zone boundaries while looking at one map series, but still allow a different series to be displayed with its native system.  (To maintain reasonable accuracy for the coordinates, this will only convert between two UTM zones when they are adjacent.)
    Whenever reasonable.
Occasionally, you may want to compare maps that have different coordinate systems.  This choice will ensure that all map data will be converted to the current coordinate system when loaded.  This way the relative positions of points will always be consistent from map to map.  It will, however, slow down the loading of those maps which need to be converted.  (To maintain reasonable accuracy for the coordinates, this will only convert between two UTM zones when they are adjacent.)


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