The first step is to set the datum and projection as described above. All site coordinates must correspond to the datum specified here. This tab also includes several data entry and formatting options
Hemisphere selection
Internally, the program uses the standard convention of positive latitudes in the northern hemisphere and positive longitudes in the eastern hemisphere. The user has the option of setting any hemisphere as positive as a data entry convenience. When site data is imported into the program, the hemispheres are defined in the import procedure. Latitude format
If the latitudes are positive in the northern hemisphere, a minus sign before the degrees will change the entry to a southern latitude. If the southern hemisphere is selected, the opposite is true. Alternately, the letters `N' and `S` can be used to specify a northern or southern latitude respectively.
Note that the latitude hemisphere selection also affects the zone in the Universal Transverse Mercator projection. The redundancy between UTM coordinates in the northern and southern hemispheres is removed by specifying the zone as north or south (e.g. 10N). A negative zone number has the effect of specifying the opposite hemisphere in the same manner as a negative latitude.
Northern Hemisphere Selected Southern Hemisphere Selected
Data Entry Result Data Entry Result
49 32 24 49 32 24.0 N 49 32 24 49 32 24.0 S
-49 32 24 49 32 24.0 S -49 32 24 49 32 24.0 N
49 32 24 S 49 32 24.0 S 49 32 24 N 49 32 24.0 N
Longitude Formats
If the longitude is set to the western hemisphere, a minus sign before the degrees will change the entry to an eastern longitude. If the eastern hemisphere is selected, the opposite is true. Alternately the letters `E' and `W` can be used to specify a eastern or western longitudes respectively.
Western Hemisphere Selected Eastern Hemisphere Selected
Data Entry Result Data Entry Result
115 32 24 115 32 24.0 W 115 32 24 115 32 24.0 E
-115 32 24 115 32 24.0 E -115 32 24 115 32 24.0 W
115 32 24 E 115 32 24.0 E 115 32 24 W 115 32 24.0 W
Precision
Geographic coordinates can be formatted to the nearest second, nearest 0.1 second or the nearest 0.01 second. This does not affect the calculations, because the coordinates are saved as double precision numbers.
Latitude Longitude Data Entry Format
Latitudes and longitudes are entered in degrees, minutes and seconds separated by space, or in decimal format or any combination of these two methods. The following examples illustrate the format:
Data Entry Format Geographic Defaults Settings
49 049 00 00.00 N northern hemisphere - nearest 0.01 second
49.5 049 30 00 S southern hemisphere - nearest second
49 30 01 049 30 01.0 N northern hemisphere - nearest 0.1 second
49 30.5 049 30 30.00 S southern hemisphere - nearest 0.01 second
49 049 00 00.00 S northern hemisphere - nearest 0.01 second
49.5 049 30 00.00 N southern hemisphere - nearest 0.01 second
122 122 00 00.00 W western hemisphere - nearest 0.01 second
122.5 122 00 00 E eastern hemisphere - nearest second
122 12 02 122 12 02.0 W western hemisphere - nearest 0.1 second
-122 14.5 122 14 30.00 E western hemisphere - nearest 0.01 second
Advanced Site Coordinate Options
Reference latitude and longitude coordinates to WGS84
The following example illustrates the use of this option
Projection British National Grid
Datum Ordnace Survey Great Britain 1936
The initial grid coordinates are X = 540000, Y = 235000 meters
The corresponding latitude and longitude are 51 59 43.25N and 00 02 21.85E. Note that these coordinates are referenced to the OSGB36 datum.
Suppose now that these coordinates were inadvertently used assuming a WGS84 datum. The error would be 113.6 meters in X and 47.7 meters in Y.
If the "Reference latitude and longitude coordinates to WGS84" option is checked, then a two step conversion is used from latitude - longitude (WGS84) to British National Grid XY. The intermediate latitude longitude in OSGB36 would not appear.
Latitude Longitude - to grid coordinate calculation sequence.
If the user enters the latitude and longitude, the projected grid coordinates are calculated and vice versa. Note that the calculation uses the major and minor axis of the ellipsoid.
Suppose the user has taken the UTM eastings and northings for a series of sites from a topographic map and entered these into the program. The user then notices that the datum was incorrectly set and therefore the latitude and longitude values are wrong. To salvage the work, the user would set the calculation sequence to "Recalculate the latitude - longitude from the projected XY coordinates" and then change the datum.
Projected coordinate system designation
This option specifies the grid coordinate labels in data entry forms and reports.
The easting and nothing designations are unambiguous; however X and Y designations can have opposite meanings on some projections. For example, in the Swiss National Grid system, X corresponds to northing and Y to easting.
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