Contents


EPSG Grids


Background

OGP, the international association of Oil and Gas Producer's Geomatics Committee, previously known as the Surveying & Positioning Committee, was formed in 2005 by the absorption into OGP of the now-defunct European Petroleum Survey Group (EPSG) which itself had been formed in 1986. The Surveying & Positioning Committee comprises specialists from OGP member companies working in the areas of surveying, geodesy, cartography and geo-spatial information/coordinate data management. The EPSG Geodetic Parameter Data set is a structured data set of Coordinate Reference Systems and Coordinate Transformations, accessible through a data registry. 

http://www.epsg-registry.org/

The geographic coverage of the data is worldwide and very complete. The EPSG Geodetic Parameter Data set is maintained by the Geodesy Subcommittee of OGP. The EPSG Geodetic Parameter Data set, offered through this OGP web registry service, may be used free of charge.  AirPhotoSE contains a copy of the entries from this data set which have needed constants.

Note:

If you only need UTM, Decimal Latitude/Longitude or Latitude/Longitude in Degrees, Minutes and Seconds, there is no need to use the EPSG database, since these are directly implemented in AirPhotoSE.  The UTM and Lat/Lon selections use the WGS84 datum which is normally the default in GPS receivers.  Many national mapping services use other datums and ellipsoids for historical reasons, but some of these are equivalent to WGS84 (e.g. GRS80, ETRS89) at the accuracies needed here. 

Accessing the data:

In AirPhotoSE Options, click the EPSG button.  If you need "world file" output for ArcGIS or an equivalent, also click the ArcGIS button.



From the main menu, click the EPSG menu item.  



Choosing Select Grid will display a window showing an area of use and an EPSG selection code.  You may choose either by area or by EPSG number. Clicking Details will show more information about the area.

 

In the window you can navigate to an area of interest either with the scroll bar or by entering the first letter of an area name in the space at the top of the window.  This is usually faster.

There are more than 3600 grids which may be selected.  They are sorted alphabetically by country or area. Each selection ends with a unique EPSG number. for example 27700  for the most recent version of the British National Grid.

You can see details for the selection by clicking on the Details button if your computer is connected to the Internet. 



This also displays a rough map of the world showing a red rectangle with the area covered by the selection.  The server which displays this map is very slow to respond, so be patient.



You can access the URL directly from your browser by pasting the link directly into the search field:

http://spatialreference.org/ref/epsg/

Because there are so many possible similar-looking choices among the thousands of grids which have been created in more than two centuries of map making, please consult the web site of the mapping agency responsible for your area of interest to verify that you have chosen the correct grid and map scale.  For the agencies listed in the GeoPortal data option of AirphotoSE, this will usually be listed in the native language of the site and in some cases in alternative languages.  

When you are satisfied that you have made the correct choice, click the Select button 



If you have made a mistake, simply reopen the EPSG selection window and change your selection, re-clicking on the Select button again when you have found the appropriate entry.  The currently selected EPSG code will be shown on the status bar at the lower right.

The parameters needed for computing grid values relative to position in either an orthophoto or map from a geoportal or another source will be recorded in the user section of the Windows Registry and used in AirPhotoSE until they are changed.  The EPSG number will also be recorded in the file made when calibrating an image using the Calibration option in AirPhotosSE or when saved from AirSE's Google Earth implementation..  These parameters apply for a given user name.  A user with different name can have a completely different set of constants.

Unless you change your area of interest or change the type of map or orthophoto used for recording and georeferencing, you only need to choose an EPSG number once.

If you open an image with a calibration file made using a different EPSG number, you will see a warning message showing the EPSG number in the calibration file and the number selected in AirPhotoSE.  



If this happens, close all images, thus returning to the main AirPhotoSE window, select EPSG again and then select the EPSG grid number corresponding to that in the calibration file of the image which showed the warning message.

Converting between EPSG and UTM-WGS84 grids:

If you have calibrated an image with an EPSG grid, or if you have an image calibrated using the UTM-WGS84 grid, you can convert between them using one of two entries on the Calibration menu: 



WGS84-UTM is available in most GPS receivers and is valid over the whole Earth, and for most of the grids in the EPSG database, there are conversion constants available.

First however, you must make certain that you have previously chosen the desired EPSG grid before opening any images.  Clicking on the appropriate entry in the calibration menu will  change it immediately.  When you close the image file, it is recommended that you change it's name. In any event, you will be warned that calibration has been changed and asked if you wish to save it.

These options may also be used to convert between EPSG grids which are valid for the same area. Consult the "Details" option when choosing the grids to be sure.  First, convert the image to WGS84 UTM, and change it's name using the Save As option. Close all images and choose EPSG in the Options-Setup dialogue, and then choose the desired new grid.  Open the image saved with the WGS84-UTM grid, and click on the WGS84 to EPSG menu entry, and save it again.  

Depending on the accuracy of the constants for transformation between EPSG and WGS84 grids supplied in the EPSG database, there may be small differences in observed coordinates.  If an image has already been converted, then you will see an information dialogue informing you of this, and nothing will be changed.  If you are saving from WGS84-UTM to EPSG and the calibration points of your image lie outside the valid area for the EPSG grid chosen, no changes will be made either.