Guilhem Bonnefille <guilhem.bonnefi...@gmail.com> writes: > 2011/5/14 Greg Troxel <g...@ir.bbn.com>: >> >> I haven't tried geotiff, but I do have DRG images for my region (in >> NAD27). If you could send a note how you configure that, I'll try it. > > I don't know what NAD27 is. AFAIK, currently, viking only support 3 > projections: UTM, Mercator and LatLon (I don't have corresponding > codes as I' writing).
Those are projections, which are ways to convert coordinates in lat/lon to numbers on some plane. NAD27 is a datum, which is a separate concept, about how particular lat/lon coordinates are assigned to particular places. Today, WGS84 is used for most things, and it's ~= to ITRF00 at the few mm level. There is also NAD83, the official datum of the US, which is a few meters different, and other datums in other places around the world. NAD27 is from 1927, and coordinates differ by tens of meters. It was the official datum of the US before NAD83, and many USGS topo maps are in NAD27. Plus NAD27 uses a different ellipsoid. So one has to transform to/from WGS84 to NAD27 to use USGS DRGs with GPS data. What I don't understand is how this ever worked. I wonder if the GPS data had been transformed to NAD27?? Perhaps gpsbabel can do that. Long term, viking should be projection and datum aware. >> I wonder if viking should depend on proj. > > I think so. I started to look a the proj4 library, but it is certainly > a significant change, reclaiming long time invest. And I don't have > this time currently. But proj support will certainly be a great add. > Anynbody working on this would gain all my support. > > Note that I started to wrote notes about this on the following wiki page: > https://sourceforge.net/apps/mediawiki/viking/index.php?title=Idea:gdal > > Feel free to contribute (on code or on this page). I have used proj, and it's pretty easy. The key question is if it's ok to depend on it, which requires anyone using viking to have it. But I think most geo software does already. I am generally in favor of requiring it, but I think we should understand/plan a bit better first.
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