The GeoEdit object comes in two forms. The first is the minimal form and contains only the controls for setting the georeferencing system (e.g. UTM, Long/Lat), while the second form also includes the georeferenced bounds of the region being described. Both forms allow viewing and editing the projection definition.
There are also two modes for the GeoEdit control. In some places it is used in ``Informational Only'' mode and none of the values may be changed by the user. Normally it is in ``User Controlled (affects loading)'' mode and any of the values may be changed interactively.
The georeferencing coordinate system is set by selecting the desired coordinate system from the option menu of possible systems. The currently available coordinate systems are:
To the right of the earth model button is a text field which contains the full georeferencing string currently in use. This includes the georeferencing coordinate system set on the option menu, plus a zone number and earth model where applicable.
The georeferencing string can be typed in by the user in the text field or can be built up from the selected georeference system, zone, earth model.
The zone number is placed after the georeferencing system, and before the earth model. The earth model appears at the end of the georeferencing string, and is indicated by a numeric code such as ``E000'' (Clark 1866 / NAD27) or ``E012'' (WGS 1984). The default earth model is ``D000'' (WGS 1984).
Each time the georeference string is changed, it is validated and reformatted. Note that changing the georeferencing system from one projection to another will not cause the bounding rectangle to be reprojected. Instead the corner values will be reinterpreted in the new georeferencing system. For a detailed list of possible georeferencing systems, earth model codes, and zone codes, consult the Projections help file.
The projections listed in the option menu are ones where all the parameters required for the projection are defined in the georeference string.
Examples:
``UTM 11 E012'' ``SPCS 1101 E000'' ``LONG/LAT E008''Other projections require more information to define the projection. When ``Other'' is selected from the option menu, a window with a list of supported projections is displayed. After selecting the desired projection from the list and pressing the Accept button, a ``Projection Definition'' window is displayed where the parameters required for the projection can be entered. Fields not required for the projection are not active.
To the right of the georeference text field is the projection report button ``More...''. It is deactivated for georeference systems with no projection information (e.g. ``PIXEL''). When active and clicked on, it it will pop up the existing projection definition. If the projection is ``UTM'' or ``SPCS'', clicking on ``More...'' brings up the appropriate Zones window.
On GeoEdit controls that are used to define a region, there will appear five additional text fields. The two labelled ``Upper Left'' define the horizontal and vertical position of the top left corner of the region. The two labelled ``Lower Right'' define the lower right corner of the region being defined.
The Bounds option menu allow the bounds to be entered as Geocoded or Geographic. Geocoded means interpret the fields in the georeference units. For example, UTM (or most projections) would be eastings and northings in metres. Long/Lat coordinates may be specified in Degrees, Minutes, Seconds form (e.g. 152d30`00.00" W or 152 30 0 w or -152 30 0) or in decimal degrees (e.g. -152.5).
Geographic means the bounds are interpreted in the equivalent Long/Lat values. If the projection parameters have been defined, the bounds can be specified or displayed as either Geocoded or Geographic.
Note that the upper left corner specified is in fact the upper and left edges of the upper left pixel for raster images, and the lower right corner is the lower and right edges of the lower right pixel.
See Also: Projections, File Utility