Projections, Datums and the GeoResGlobe
Spatial data and spatial systems work with information collected from a flattened sphere. The data can be captured in a geographic coordinate system which plots data on a 3D spheroid measure in degrees of latitude and longitude, or a projected coordinate system where the data is plotted on a flat surface (a computer or phone screen, or a hardcopy map). Both these systems have to reference a datum which provides the foundation and reference for measurements by defining the general shape of the earth. Different datums have been defined, from local datums through to some that best fit the entire earth.
Geographic coordinate systems are often used to work with data at whole of state or national scales, whereas some specific projected coordinate systems such as MGA2020 use Eastings and Northings (measured in metres) and are often used at local or regional scales. All projected coordinate systems have to distort some element on a map – It is not possible to preserve the shape, area, distance, and direction of features on the earth over a large area when looking at a flat representation of the earth. Therefore, it is generally considered that there is no perfect map projection, only the most suitable projection for an application.
|Geocentric Datum of Australia 1994||GDA94||The Geocentric Datum of Australia 1994 was the official geodetic datum before GDA2020. This has been replaced by GDA2020 (below).|
|The Geocentric Datum of Australia 2020||GDA2020||The Geocentric Datum of Australia 2020 became the national geodetic datum for Australia in December 2017.|
|World Geodetic System 1984||WGS84||World Geodetic System 1984 is the reference frame used by the Global Positioning System (GPS).|
|Map Grid of Australia 2020||MGA2020||A Universal Transverse Mercator projection using GDA2020 datum. Eastings and Northings, suitable for use in MGA Zones (multiple zones are used across Australia).|
|Map Grid of Australia 1994||MGA94||A Universal Transverse Mercator projection using GDA1994 datum. Eastings and Northings, suitable for use in MGA Zones (multiple zones are used across Australia).|
|Web Mercator||Google Web Mercator, Spherical Mercator, WGS 84 Web Mercator, and Pseudo-Mercator.||Web Mercator projection uses the WGS 84 datum. It has become the de facto standard for web mapping. Web Mercator is used by the Qld Globe, GeoResGlobe, Google, Bing, OpenStreetMap and others.|
The GDA2020 datum is an example of what is known as a static datum. As continents physically “drift” very slowly around the world, various static datums such as the GDA94 and GDA2020 datums have been defined at these points in time. The difference between the same point measured on the 2 different datums - MGA94 and MGA2020 Easting and Northing coordinates - is approximately 1.8 metres.
The GeoResGlobe uses a Web Mercator projection. The spatial data seen in the GeoResGlobe can be stored in any coordinate system. However, to ensure the different data sets align correctly relative to each other, they are all projected into the common Web Mercator projection on the WGS84 datum for display purposes. Behind the scenes, the application software reads the coordinate system of the source data and transforms the display of that data to WGS84 Web Mercator on-the-fly. This built-in capability allows GeoResGlobe to view data features at their correct location relative to each other. The coordinate output and measurement tools provided in the GeoResGlobe are “coordinate system aware” and are able to display the actual coordinates of features in a number of user selected coordinate systems, including the commonly used GDA2020.
GDA2020 Datum and the GeoResGlobe
Various static datums such as the Australian GDA94 and GDA2020 datums have been defined as basically equivalent to WGS84 at the (different) date when each datum was defined. As accuracy needs intensify into the future, it is understood that future solutions will need to understand the date (epoch) when a coordinate was captured on an individually drifting continent. Therefore, these static datums have a limited life. Dynamic datums are coming and are expected to solve these types of issues, however it requires the spatial industry around the world to put its mind to the problem of implementation from a world-wide context. Whilst this is starting to be considered, it is a few years away from resolution.
With this in mind, it is important to understand the GeoResGlobe is a web mapping application utilising web services with data displayed in the Web Mercator projection on the WGS84 datum. For all Departmental data displayed in GeoResGlobe, the application software reads the coordinate system of the source data and transforms the display of that data to WGS84 Web Mercator on-the-fly. This functionality allows GeoResGlobe tools to display data features at their correct location, relative to each other in Web Mercator, and display the actual coordinates of features in the chosen coordinate system using the coordinate output and measurement tools (i.e., the same transformation in reverse).
The application supports the adding of data and layers from either GDA94 or GDA2020 datums which will draw appropriately relative to each other and/or background imagery. The Draw Places and Coordinate tools will place features at their correct location and display the actual coordinates of features by selecting the cursor output options relative to the chosen datum.
However, for printed map output, the map is faithfully reproduced from the image on the screen, i.e., the Web Mercator projection using WGS84 datum. Due to limitations in the Web Mercator projection precise measurements off the hardcopy printed output are not advised.
GDA2020 Source Data
While the Web Mercator Projection creates distortion when depicting data in GeoResGlobe, the source data is stored without those distortions in a rigorous coordinate system, such as GDA2020. For applications requiring more accurate measurements of length or area, it is possible to obtain the GDA2020 source data from QSpatial and use it in more rigorous software such as a Geographic Information System (GIS).
Worldwide, online mapping systems make extensive use of the Web Mercator projection based on the WGS84 datum to display the surface of the Earth. Its strength is preserving direction and shape that enables a decent level of relativity between features anywhere around the world. However, this approach comes at the expense of retaining the actual distances and areas of the features. Consequently, it is not necessarily an accurate approach in all circumstances.
For all Departmental data displayed in GeoResGlobe, the application software reads the coordinate system of the source data and transforms the display of that data to Web Mercator on-the-fly. This functionality allows GeoResGlobe tools to display data features at their correct location, relative to each other in Web Mercator, and display the actual coordinates of features in the chosen coordinate system using the coordinate output and measurement tools (i.e., the same transformation in reverse).
However, the weakness of Web Mercator is in retaining the actual distances and areas of the features, and this becomes evident when creating Printed Maps. The method currently employed in creating the Printed Map output in the GeoResGlobe faithfully reproduces from the image on the screen and does not remove the distortions typical in the Web Mercator projection.
Consequently, the use of the GeoResGlobe for the purposes of precise measurement is not advised. Please refer to the Disclaimer notice on the application’s Welcome page for further details.
Adding GDA2020 Data to GeoResGlobe
What is the issue?
GeoResGlobe is a web mapping application based on web services with data displayed on WGS84 datum using the Web Mercator projection. It is not currently possible to use Coordinate Reference System metadata (e.g. EPSG codes) to distinguish whether WGS 84 Web Mercator values are based on GDA94 or on GDA2020. This issue is described in detail in the ICSM paper entitled GMIWG Advisory on WGS 84 and Web Mapping.
How is this handled in GeoResGlobe?
Until the longer term solution described in the ICSM paper can be implemented, GeoResGlobe will continue to display data using GDA94 based WGS84 Web Mercator. For all Departmental data displayed in GeoResGlobe the software is able to read metadata to identify whether the source data is in GDA94 or GDA2020 and convert to GDA94 based WGS84 Web Mercator on-the-fly. Similarly, when using the Add My Data feature, it is necessary to supply metadata specifying whether it is GDA94 or GDA2020 data.
The Add My Data feature in the GeoResGlobe supports the import of simple GPX, KML, KMZ, SHP or CSV files. However, it should be noted that only SHP files allow the specification of the datum. That is done by including a PRJ file in the Zip file submitted using Add My Data.
For all other data formats, the datum will be assumed to be in GDA94. Therefore, if you have GDA2020 data in one of those other formats you will first need to transform the data back to GDA94 before importing it into GeoResGlobe.
The following illustrates how various files types will be plotted relative to other data sets in GeoResGlobe:
|File Type||Datum||Plotted relative to other GeoResGlobe layers|
|SHP (with PRJ)||GDA94||Plots correctly|
|GDA2020||Plots correctly (PRJ allows transform to WGS84 based on GDA94)|
|SHP (without PRJ)||GDA94||Plots correctly (without PRJ defaults to WGS84 based on GDA94)|
|GDA2020||Plots incorrectly (defaults to WGS84 based on GDA94)|
|Files other than SHP format||GDA94||Plots correctly|
|GDA2020||Plots incorrectly (defaults to WGS84 based on GDA94)|
What about other Web Mapping?
It should be noted that many other existing web mapping in Australia are likely to also be using WGS84 Web Mercator based on GDA94 and users should clarify with the provider how best to handle GDA2020 data in those applications.
Where can you get more information?
For more information on GDA2020 in Queensland go to GDA2020 in Queensland | Department of Resources
The website has information on the adoption of GDA2020 in Queensland and includes links to documents on GDA2020 impact assessment, technical background and transformation processes and software. There are also links to the ICSM web site, which is the best source of national information on GDA2020.
If you require any additional information on specific topics please contact us on GeodeticSupport@resources.qld.gov.au