Dennis Lee Bieber wrote: > On Wed, 10 Oct 2012 10:15:58 +0100, Ian Goddard<goddai01@hotmail.co.uk> > declaimed the following in soc.genealogy.computing: > >> pblair wrote: >>> Be safe. >>> >>> I add a latitude and longitude (which seem to have been reasonably stable over the years) >>> >> >> I'm beginning to wonder about that, and I don't mean continental drift. >> >> I've just spent several days trying to georeference maps using QGIS& >> failing to get consistency between coordinates obtained by different means. > > Well... You have to take into account the datum used... There is a > difference between NAD27 (okay, I'm US biased -- most USGS maps still > use NAD27), NAD83 (short-lived I believe), WGS-72, WGS-84, UTM... And > for the UK, the OSGB grid system, etc. > > I'm in the UK. But a 1st edition 6" map is the best part of a century older than the current coordinate system. The maps use Cassini rather than Mercator (and, according to information received, have different meridians for different counties). This means that either the old maps have to be used as is with an arbitrary set of coordinates which are simply pixel coordinates or reprojected onto OSGB. Although individual sheets have lat/long grids on the edges without knowing the parameters of the original projection the only alternative is to use a lot of reference points. I can use a plugin for Google Earth to give coordinates in OSGB which is one alternative. I can also use the OpenLayers plugin to show Google Maps hybrid layer and on-the-fly reprojection to OSGB. And these two give results which differ by a hundred yards or so. And trying to overlay the downloaded OS district vector images on Google Maps introduces a whole new level of weirdness. I can also use the raw lat/long data from GE or GM & use some calculator to calculate the OS coordinates. And the calculator on the current stable Linux release of QGIS gives different answers from the previous stable or the current development or the current Windows version. In fact I've used the highest zoom level of Streetmap which I believe is based on OS data but represents buildings as simplified polygons. The features I've relied on to be unchanged over a century are mostly churches and the simplified polygons are simple rectangles but almost all the churches are irregular with porches and/or chancels which the original shows; in one case it even represents the buttresses. So does the rectangle simply represent the nave or does it enclose the whole irregular outline? In short I have a reprojection of sorts and in the middle of the map it fits the downloaded OS District raster reasonably well allowing for the fact that the much poorer resolution of the latter but near the edges, even at control points, the coordinates are out by several tens of metres. -- Ian The Hotmail address is my spam-bin. Real mail address is iang at austonley org uk