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Projections and Coordinate Systems

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Most of the output formats supported by MPExporter are geographic formats that use the same geographic decimal longitude,latitude WGS84 coordinate system as MapPoint. The exceptions are SVG and Excel, and MPExporter can create these formats using different coordinate systems.

 

SVG is a non-geographic spatial format, i.e. intended for drawings on a flat surface. In order to do this properly and in the most flexible manner,  MPExporter supports a range of map projections and coordinate systems.

 

The Excel format simply writes the coordinates to columns in the output worksheet(s). Although most users will probably use geographic coordinates, MPExporter also supports the use of other map projections and coordinate systems. These can make it easier to transfer the data to other applications, or to compare them with existing geospatial databases.

 

For both formats the coordinate system is selected using the options in the Coordinate System box:

 

mainpanel_coords

 

MPExporter can export to SVG using one of four different projection and coordinate system options:

 

Geographic Coordinates on the WGS84 Datum

UTM (Universal Transverse Mercator)

A Predefined Coordinate System

An External PRJ file

 

The "Geographic Coordinates on the WGS84 Datum" is simply the MapPoint standard coordinate system. This is usually the default. Selecting one of the remaining three options will enable the button in the lower right. The exact caption on this button will vary according to the option.

 

The "UTM (Universal Transverse Mercator)" option will allow you to select a UTM Zone for the output coordinate system. The UTM coordinate system has 60 zones and two hemispheres. Select the required zone and hemisphere. MPExporter also lets you select one of four datums (WGS72, WGS84, NAD27, NAD83). MPExporter does not support the corresponding Universal Polar Stereographic (UPS) systems, due to the poor handling of near-polar coordinates by MapPoint. Note: MPExporter will only successfully export coordinates that are within the selected zone and one adjacent zone on each size. Any coordinates outside of this area will be skipped, potentially resulting in malformed shapes.

 

Press the Select Zone button to display the Select the required UTM Zone Parameters dialog box:

 

export_utm

 

The 'Predefined Coordinate System' option allows you to select a coordinate system which has been predefined in MPExporter. Press the Select System button to display the selection dialog box:

 

predefined_coords

 

MPExporter currently has the following coordinate systems pre-defined:

 

Geographic Coordinates using the WGS84 Datum

British Ordnance Survey National Grid using the OSGB36 Datum

Geographic Coordinates using the OSGB36 Datum

 

The final 'External PRJ File' option allows you to specify a custom projection and coordinate file using an external 'PRJ' file. Select this option, and then press the Select PRJ File button to select the PRJ file.

 

Here is an example of a US map (population by county) plotted using an Albers Equal Area Conic (ESRI:102008) projection which has been provided with a PRJ file:

 

sample_us_albers

 

 

SVG Specific Coordinate Options

 

MPExporter also provides a number of SVG-specific drawing options related to coordinates. First there is the Line Thickness in the Shapes box. SVG thicknesses and sizes use the same coordinate system as the actual drawings. This contrasts with KML and the HTML options which define object sizes in pixels whilst positions are in geographic coordinates. Therefore we have to specify line thicknesses in terms of map coordinates. For example, if you are plotting geographic coordinates (degrees), then the line thickness is defined in terms of degrees. Typically in this scenario, you will want to use a line thickness that is a fraction of a degree, perhaps 0.01. The line thickness is also used to define the size of pushpin images (size: 16 pixels) and text. The line thickness is applied before the scale (see below).

 

After applying the projection, MPExporter can transform the coordinates into a coordinate system more acceptable for your final application. For example, most map coordinate systems result in very large coordinate values — typically 100,000s or even 1,000,000s. Many SVG packages cannot handle such large coordinate systems. Also, many packages (especially web browsers) expect a coordinate system defined in pixels with the origin in the top left corner. This final transformation is set using the Final Coordinates option. This can be set to Scale Coordinates to a Window or Transform and Scale Coordinates.

 

Scale Coordinates to a Window is the easiest to use, but it does not automatically preserve the aspect ratio. Simply specify the required window in output coordinates. The Origin defines the upper left corner. The Max X, Max Y values specify the lower right corner. For a web browser application, you would set the origin to 0,0; and then set the Max X and Max Y values to the required width and height in pixels. MPExporter will then stretch/squash the resulting map to fit this window. Note: This option will result in MPExporter running through the data twice. The first time is to determine the extents, the second is to perform the actual export. This could take a long time for a complex map.

 

The Transform and Scale Coordinates option is guaranteed to preserve the map's aspect ratio but it tends to be more difficult to use. This option applies a scale factor and then follows it with a translation. Set the scale factor using the Scale (in the Coordinate System box). This defines a multiplier that is applied to all post-projection. After applying the scale, MPExporter applies a translation as defined by the origin (Origin (X) and Origin (Y)). This moves (translates) the coordinates into a useful area of the coordinate system.