Coordinate reference systems

In a map, the three-dimensional earth is represented two-dimensionally, and the needed transformation can be made using different models. Each model expresses locations on earth in coordinates in a coordinate reference system. Spotfire supports more than 3000 of these systems.

Note: You can update the coordinate reference system to use in a specific map chart or layer using the Project to and Project from sections in the visualization properties panel. Specifying default settings for coordinate reference systems can only done in the installed client.

Geographical coordinate reference systems define points on earth's surface by their longitude and latitude values. The values are angles measured from the center of the earth with the zero longitude set to a certain meridian, and they form the coordinates in the coordinate reference system.

Another option of transformation is projecting the earth onto a flat surface. Locations are then transformed into x- and y-values in a coordinate reference system. Different projecting models exist, resulting in different Projected coordinate reference systems.

Visualizing data in a map chart requires two specifications of coordinate reference systems, both explained below.

Coordinate Reference System for a data layer

When you create a map chart, you position data in a geographical context using coordinate columns available in your data table or using geocoding. In either case the data needed for the positioning is expressed in a certain coordinate reference system. For each added layer with data to position, you specify the coordinate reference system in question in the Data section of the layer's visualization properties (click Settings for the layer of interest), or, using the Project from section in the visualization properties panel.

Note: The geocoding data tables provided by Spotfire are expressed in the EPSG:4326- WGS84 coordinate reference system.

Note: If you are going to use an image as a background, then the coordinate reference system must be 'None' in all parts of the map chart visualization. This includes all layers in the map chart where a coordinate reference system can be specified, as well as the map chart itself. See Positioning markers on an image background for more information.

Coordinate Reference System for the map appearance

The projection model used for the appearance of the map, on top of which markers and features should be positioned, is also represented in a certain coordinate reference system. This coordinate reference system is specified on the Appearance page of the visualization properties dialog, or under Project to in the visualization properties panel. This means that different coordinate reference systems can be used in a single map chart. You can use one system for the map's appearance and another system for a data layer. You can also decide to Show datum shift warnings if there are issues with datum shifts. There are cases when it is not possible to apply a datum shift to the map chart if coordinate reference systems with different datums are used. This can lead to a lower accuracy in the map chart.

Note: If you want to use Map Layers in your map chart, the Coordinate Reference System for the map chart must be set to EPSG:3857-WGS 84 / Pseudo-Mercator.

Below are examples of identical data layers (using EPSG:4326- WGS84 as coordinate reference system) that are added to map charts with different coordinate reference systems set on the map chart:

When another coordinate reference system than the one used for the data layer is chosen, a recalculation of the coordinates in the data layer takes place to enable correct positioning.

When configuring the map chart, you should consider which coordinate reference system that suits your needs best. All projection models cause, more or less, different types of distortions of shapes, areas, distances and directions. For example, one model might represent a certain geographical area well (like a country), but if you move away from this particular area to another part of the map, the same acceptable appearance might not be preserved. That is, you should select a coordinate reference system with minimal distortions for your particular purpose of the map, and that works well for the geographical region you want to include in the map.

Note: If your map chart contains an image layer with an image that will be used as a background, then the coordinate reference system must be 'None' in all parts of the map chart visualization. This includes all the layers in the map chart where a coordinate reference system can be specified, as well as the map chart itself.

Adding another coordinate reference system

You can select a coordinate reference system that is not available in the drop-down list of the properties for the map chart by clicking Other in the visualization properties for the map chart (or Settings for the layer), or by clicking Add reference system in the drop-down lists of the Project to or Project from sections in the visualizations properties panel.

You can also specify the default coordinate reference systems to show for new map charts and layers on Tools > Options > Map chart (installed client only).

Depending on from where you open the dialog, it looks slightly different.

You can use search to locate a coordinate reference system in the list.

The Coordinate system/ID list shows all the available coordinate reference systems including their spatial reference ID (EPSG code). The coordinate reference systems (CRS) are categorized as either geographical, projected, or imported. Select the CRS you want to use for the map chart or the individual layer.

See tooltips for details on each listed CRS.

Note: Imported coordinate reference systems appear in the list if you have added a shapefile or GeoTIFF image whose CRS definition is not already included in the list of available coordinate reference systems. Spotfire will then attempt to use the CRS definition in the added file's linked projection information instead, and the CRS will be listed as a 'Generated CRS'.

Projection parameters (Proj4) (only available in visualization properties) lists the Proj4 parameters for the selected coordinate reference system, including the projection name and datum shift description, when applicable. For detailed information about projection parameters, refer to the official Proj4 documentation.

Note: For GeoJSON files, EPSG 4326 is the required coordinate reference system.

When you open the dialog from the Map Chart page in Options, you can select the check boxes for all coordinate reference systems you want to show in the drop down-list in the properties for the map chart or the individual layers, and click Set as Map Chart Default or Set as Layer Default, respectively.