On second thought, it occurs to me that it is *Google Maps* that is placing the marker down onto the tiles rendered by TerraServer. All TerraServer receives from Google Maps are the four points bounding the rectangle to be rendered. TerraServer must place these four points down onto its TIFF image and return a *rectangular* set of tiles *approximately* bounded by the four points (for they will not in general form a rectangle in its UTM projection). Google Maps will then assume that the marker should be placed in the very center of the image rendered by TerraServer. Although the fact that I'm making rather high resolution requests should minimize projection errors (as should the fact that the marker is in the center of the requested tiles), I am dependent on the methodology employed by TerraServer to "fudge" this bounding rectangle. I would also wonder whether marker locations might move slightly with respect to features visible in the TIFF as the map is scrolled around.
On Oct 22, 7:55 am, Chris Schneider <[email protected]> wrote: > Thanks for the quick response, Rossko! > > I'm no expert either, but the problems described in these two links > would seem to be dependent on the scale of the maps being rendered. As > John Deck admits in his text, "The interesting thing is that if we > were to zoom in on a particular portion of the world, the boundaries > would line up." Since I'm rendering maps at such high resolution, I > would not expect the projection conversion being done by TerraServer > (fromEPSG:4326 to UTM) to introduce the positional error I am seeing. > More importantly, my application does not require that four such > points be rendered as a rectangle by both engines. > > That said, I am expecting my latitude and longitude coordinates to be > rendered properly by Google Maps as well. As mentioned in my original > email, these coordinates come from the USGS Geographic Names > Information System (GNIS), which claims them to be NAD83. I had > expected that such coordinates would be placed correctly when sent > directly into the Google Maps APIs, and when sent to the TerraServer > asEPSG:4326. > > On Oct 22, 5:16 am, Rossko <[email protected]> wrote: > > > Might be worth > > followinghttp://lists.osgeo.org/pipermail/webmap-discuss/2006-September/000154... > > andhttp://johndeck.blogspot.com/2005/09/overlaying-mercator-projected-wm... > > > I'm no expert but it seems to me that sending WGS84 coords and calling > > themEPSG:4326 without doing any actual conversion will lead to the > > kind of inaccuracy you describe? Those sources seem to suggest > > thatEPSG:41xx would be closer, if still inaccurate? --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Google Maps API" group. To post to this group, send email to [email protected] To unsubscribe from this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/google-maps-api?hl=en -~----------~----~----~----~------~----~------~--~---
