Hello GLIMS-related folks. I submit the following for your feedback. I am sending it to the entire community because 1) the issue touches the whole community, and 2) this list has too little traffic on it. :-) Happy New Year. Bruce Issue: coordinate systems used in GLIMS Previous idea: Multiple coordinate systems would be supported by (stored in) the GLIMS database. Examples include: lat/long, locally defined northings/eastings, UTM. Benefit of previous idea: GLIMS data suppliers (Regional Centers) could work in their preferred coordinate systems and submit the data without conversion. Problems with previous idea: - End users of the GLIMS database who query the database over potentially large regions should be able to expect the returned data to be in a uniform coordinate system, not a hodge-podge of different projections. - Storing many coordinate systems complicates the design of the database. Proposal: The only coordinate system stored in the GLIMS database will be lon/lat on the WGS84 datum. Benefits of current proposal: Having a single coordinate system for the entire GLIMS database would greatly simplify its use, as well as its implementation. Potential problem with current proposal: Regional Centers must translate their output data into lon/lat/WGS84. (This is thought not to be a big problem.) Notes: - Absolute and relative positional uncertainties will still be tracked separately. - We acknowledge that many RCs do or will work in local coordinate systems (e.g. UTM/NAD83, Polar Stereographic), but we assume it wouldn't be a big burden to request that all new analysis results be submitted in lon/lat/WGS84. Is this assumption correct? - For older data, and perhaps special cases with newer data, NSIDC may be able to do the coordinate conversion at time of data ingest. But my sense is that most RCs should be able to do the translation easily with whatever package(s) they use to generate the results. - Commercial packages like ESRI's should be able to handle coordinate transformations easily. I also know of a few free software packages that will do coordinate transformation: GMT (Generic Mapping Tools (http://gmt.soest.hawaii.edu/)) Proj.4 (Cartographic Projections Library; does datum shifts as well. (http://www.remotesensing.org/proj/)) GRASS (actually uses Proj.4 internally; http://www.baylor.edu/grass/) - Distances and areas are generally calculated in projected coordinates. Obviously, this can still be done at Regional Centers. There is, however, the question of how individual points (polygon vertices) are assumed to be connected: along great circles, or as straight lines in some projection. A quick test has shown (see below) that this difference will not be a problem, assuming 1) suitable projections are used by both data supplier and end user, and 2) points that are connected aren't more than 100 km or so apart. In the below, "Error" is the distance between the midpoint of the segment as calculated on the great circle connection versus that calculated on the straight-line connection in the projection. Lengths (last two columns) are great circle lengths. Projection: South polar stereographic, true scale at -71 degrees) Input coordinates Length Error (lon,lat,lon,lat) Note (km) (km) --------------------------------------------------------------------------- -60 -75 -50 -77 Ronne Ice Shelf front 349.700 0.290 -165 -78 -180 -78 Ross Ice Shelf front 347.335 0.249 -165 -78 -166 -78 Ross Ice Shelf front 23.219 0.001 -165 -78 -169.33 -78 100 km segment, east-west 100.516 0.021 -165 -78 -165 -78.9 100 km segment, north-south 100.484 0.019 Projection: (UTM zone 13, over Colorado, USA) Input coordinates Length Error (lon,lat,lon,lat) Note (km) (km) --------------------------------------------------------------------------- -105.6505 40.0233 -105.6490 40.0251 Arapaho Glacier 0.237 0.000 -105 40 -105 40.9 100 km segment north-south 99.939 0.002 -105 40 -106.175 40 100 km segment east-west 100.337 0.002 -108 40 -108 40.9 same N-S, at edge of zone 99.939 0.008 -108 40 -109.175 40 same E-W, at edge of zone 100.337 0.010 -- Bruce Raup National Snow and Ice Data Center Phone: 303-492-8814 University of Colorado, 449 UCB Fax: 303-492-2468 Boulder, CO 80309-0449 braup@nsidc.org