Transform polygon map

Functionality / Algorithm

The Transform polygon map operation transforms the coordinates of polygon boundaries in a polygon map from the map's current coordinate system to another target coordinate system. You can choose whether coordinates of polygon boundaries should be densified before the transformation takes place.

A transformation can only be performed between compatible coordinate systems. You can:

transform a polygon map with a coordinate system projection with a certain projection, ellipsoid and/or datum to another coordinate system projection with a different projection, ellipsoid and/or datum;
transform a polygon map with a coordinate system projection with a certain projection, ellipsoid and/or datum to a coordinate system latlon with a certain ellipsoid and/or datum (and vice versa);
transform a polygon map with a coordinate system latlon with a certain ellipsoid and/or datum to another coordinate system latlon with a different ellipsoid and/or datum;
transform a polygon map with a coordinate system formula to the 'related coordinate system' of this coordinate system formula (and vice versa);
transform a polygon map with a coordinate system tiepoints to the 'related coordinate system' of this coordinate system tiepoints (and vice versa).

Furthermore:

when in a coordinate system an ellipsoid is not specified, then a sphere is assumed, and
when in either of the 2 coordinate systems a datum is not specified, then the same datum is assumed.

For more information on coordinate system types, see ILWIS object : coordinate systems.

In general, the operation can be used:

to integrate data which are obtained from different sources and when these data are in different projections,
to present data or results in a more appropriate projection.

Preparations for using a coordinate system projection:

When you want to integrate data of different projections, first think of the most suitable projection in which you can do your analysis: when area calculations are involved, you should use an Equal Area projection.
You can either:

transform your maps to one existing coordinate system; select this existing coordinate system as the target coordinate system during Transform operations.
first create a new coordinate system, specify projection information, and then transform all your maps to that new coordinate system.

When you want to present data in other projections, you can generally first create a number of new coordinate systems (using File, Create, or by clicking the create button in the dialog box of a Transform operation) and then transform your map to these new coordinate systems. During the Transform operations, specify a different output map name for each selected target coordinate system.
For presentation purposes, you can also interactively change the coordinate system as used by maps displayed in a map window. For more information, see the tips below.

For more information on available projections, refer to the Select Projection dialog box.

Densify coordinates:

During a transformation, straight polygon boundaries which are simply defined by a start node and an end node will always remain straight as only the coordinates of the begin and end node will be transformed.

However, polygon boundaries generally consist of a start node and an end node and many intermediate points in between. Then, it is strongly advised to increase the number of intermediate points in all polygon boundaries by using the Densify Coordinates option during the Transform operation, and specify a distance interval at which intermediate points should be inserted. By using the Densify Segments option, the shape of polygons will be better preserved during the transformation.

For more information, see also Densify segment coordinates operation.

Input map requirements:

A transformation is only possible between:

a coordinate system projection with a known projection, ellipsoid, and/or datum, and

another coordinate system projection with another projection, ellipsoid, and/or datum, or
a coordinate system latlon with a known ellipsoid and/or datum, or

a coordinate system latlon with a known ellipsoid and/or datum, and

another coordinate system latlon with a known ellipsoid and/or datum, or

a coordinate system formula, and

the 'related coordinate system' of this coordinate system formula, or

a coordinate system tiepoints, and

the 'related coordinate system' of this coordinate system tiepoints.

In each of these combinations, one coordinate system is the current coordinate system of the polygon map, and the other is the selected target coordinate system.

Domain and coordinate system of output map:

The output polygon map uses the same domain as the input polygon map.

The output polygon map uses the selected target coordinate system; the coordinate boundaries of the output map will be the transformed coordinate boundaries of the input map.

Tips:

To see the effect of using different projections, it is advisable to display the input map in a map window and the output maps in other map windows; then add a graticule to the map windows; open the Layers menu and select the Add Annotation, Graticule command.
The Transform operations permanently change the projection of your map(s), i.e. for analysis and calculation purposes.

To temporarily view map(s) which are displayed in a map window in another projection, i.e. for presentation purposes, you can:

create a coordinate system in which you already specify some projection information,
display your map(s) in a map window,
drag your new coordinate system to the map window, or choose the Coordinate System command from the Options menu in the map window and subsequently select another coordinate system.

The contents of the map window will be displayed in the new projection. Projection information of the coordinate system as currently used by the map window can be refined by choosing the Coordinate System command from the Edit menu in the map window. Thus, you do not need to use a Transform operation.

Algorithm:

The XY-coordinates of the polygon boundaries of the input map are copied, and then transformed.

When both the input coordinate system and the target coordinate system have the same ellipsoid and datum information, then the transformations are calculated via geographic coordinates; i.e. from XY (input) to f, l (LatLon) to XY (target).
When in the input coordinate system and the target coordinate system, ellipsoid and datum information is different, then Molodensky formulas are used to calculate the ellipsoidal transformations and datum shifts, i.e. from XY (input) to f, l (input) to f, l (target) to XY (target).

Finally, new polygon areas and perimeters are calculated.