Projection transformations can be carried out on LOC files in the LOCPREP application using the Projection menu. Forward and reverse transformations are supported, to convert Lon-Lat to X-Y coordinates, and back.
The following projections are supported:
- UTM - a standardised version of the Transverse Mercator system. The Earth is divided into 60 vertical zones each spanning 6° longitude. The central meridian for each zone is the line of longitude in the centre of the zone. The limits of UTM projections are 84°N and 80°S.
- Transverse Mercator - points on the Earth's surface are projected from the centre onto the surface of a cylinder with its axis parallel to the plane of the equator. The cylinder touches the surface of the Earth ellipsoid at the equator, at the Longitude of the Central Meridian.
- Lambert Confomrmal Conic - points on the Earth's surface are mapped onto part of the surface of a cone, between two lines of Latitude, called the standard parallels.
Projection Direction can be either LL to XY (forward) or XY to LL (reverse).
After editing the projection parameters you can select whether to perform the projection transformation on the current LOC file.
If you select YES, records will be read from the current LOC file and transformed into a new temporary LOC file which will be displayed by LOCPREP. Information about the projection transformation will be added to the LOC file headers and can be viewed using the LOCPREP Edit Headers option.
If you select NO you can use the Projection Test option to transform individual coordinates (LOCPREP Test Projection Coordinates), or save the projection as a projection configuration file (LOCPREP Save Projection File).
All projections must specify a spheroid to use. Because the Earth is irregular, different spheroids are conventionally used for different regions to provide the best fit for the Earth's surface.
Select the spheroid to use from the drop-down list in the projection configuration editor. Some common spheroids for different regions are listed below:
Spheroid Name Region Airy 1830 UK Bessel 1841 Central Europe, Chile, Indonesia Clarke 1866 North America, Philippines Clarke 1880 France, Africa Everest 1830 India, Burma, Sri Lanka Helmert 1907 Egypt Krasovsky 1940 Russia, Eastern Europe The parameters for common spheroids are listed in the [spheroids] section of PROJCONV.INI, and can be added to by users if required. Alternatively, you can supply a custom spheroid by selecting Custom from the drop-down list, and entering its name, semi-major and semi-minor axes.
Zone
A region's standard UTM projection will depend on the longitude 'zone' into which it falls. Zones are 6° of longitude wide, and are numbered from 1 (central meridian 177°W) to 60 (central meridian 177°E). Select the zone from the drop-down list.Hemisphere
UTM Zones have standard parameters for a Transverse Mercator transformation in either the Northern or Southern hemisphere. This will affect the False Northing Value used for the transformation.The standard parameters used for UTM transformations are:
- Base Latitude 0° (equator)
- False Easting 500000
- False Northing Northern hemisphere 0, Southern hemisphere 10000000
- Central Meridian determined by UTM Zone
- Scale Factor 0.9996
False Northing (m) - an offset from zero to avoid having negative coordinate values. This value is subtracted from Y-values.
False Easting (m) - an offset from zero to avoid having negative coordinate values. This value is subtracted from X-values.
Central Meridian (deg) - the longitude in degrees from which X coordinates are measured. Areal distortian is minimal along the central meridian. If the Scale Factor is 1.0, map scale is accurate along the central meridian, otherwise, if the Scale Factor is less than 1.0, then there are two straight lines having an accurate scale, equidistant from and on each side of the central meridian.
Base Latitude (deg) - the latitude in degrees from which Y coordinates are measured.
Scale Factor - an adjustment to the scale to make distances on the map, on average, more accurate. Usually 0.9996, but sometimes 1.0000 (in Egypt, for example).
False Northing (m) - an offset from zero to avoid having negative coordinate values. This value is subtracted from Y-values.
False Easting (m) - an offset from zero to avoid having negative coordinate values. This value is subtracted from X-values.
Central Meridian (deg) - the longitude in degrees from which X coordinates are measured. Areal distortian is minimal along the central meridian. If the Scale Factor is 1.0, map scale is accurate along the central meridian, otherwise, if the Scale Factor is less than 1.0, then there are two straight lines having an accurate scale, equidistant from and on each side of the central meridian.
Base Latitude (deg) - the latitude in degrees from which Y coordinates are measured.
Parallel 1 (deg)
Parallel 2 (deg) - these define the secant where the projecting cone touches the spheroid for a 2-parallel conic Lambert projection. For a 1-parallel conic Lambert projection, set both Parallel 1 and Parallel 2 to the same value, defining the tangent to the spheroid which is projected into a cone.Scale Factor - an adjustment to the scale to make distances on the map, on average, more accurate. Usually 0.9996, but sometimes 1.0000 (in Egypt, for example).
The projection test is available from the LOCPREP Test Projection Coordinate menu option. It can be used to transform individual coordinates, and to test the results of a particular projection configuration if you are uncertain of the exact projection to use.
Specify the direction for the projection test, either LL to XY (forward) or XY to LL (reverse). This is independent of the direction specified in the main projection parameters.
Then enter the input coordinates, either a Lat/Lon pair or an X/Y pair depending on the projection direction.
Then click the Next button to display the output coordinates for the specified input coordinates. You can click the Back button and enter a new set of input coordinates to test the transformation again.
See also:
[LOCPREP - Location
File Editor]