5 Jan 09:01 2015

### Time dependent coordinate transformations

Chris Crook <ccrook <at> linz.govt.nz>

2015-01-05 08:01:25 GMT

2015-01-05 08:01:25 GMT

I'm wanting open a discussion around how to handle time dependent coordinate transformations. These are becoming far more relevant to GIS data sets and positioning than they have historically been. This is a growing issue for which I'd like to find the right forums and process to ultimately have have widely supported in coordinate transformation implementations, so please suggest or forward other appropriate forums. I'm posting initially to the PROJ list as PROJ underlies much of the coordinate system transformations in the open source GIS stack, and does include datum transformations as well as projections. However this is clearly a wider issue .. as I suggest below I think this affects the definition of spatial features generally - so I'd welcome suggestions for more appropriate or additional forums. The background ============== Coordinate systems are based on geodetic datums. These are generally either international datums, such as WGS84 or ITRF realisation, or national datums defined by a country's geodetic authority. Within international datums apparently fixed features (such as buildings) are moving due to plate tectonics at rates of typically 5 cm/year. This means that the WGS84 coordinate of a "fixed" point in 2000 would now miss that point by 75cm. National datums are generally referenced to fixed marks within the country and provide a stable coordinate system, such that the coordinates of fixed marks are constant. Obviously the conversion between international datums and national datums cannot be done without taking account of when the coordinates are applicable. Typically GIS systems are in terms of national coordinate systems. Generally they contain spatial definitions in terms of the national datum, with no timestamp, and assume that this provides a consistent(Continue reading)