Wednesday, September 24, 2008

the problem of publishing geo-locations

Where 2.0, GPS, maps and genealogy - Google Search

from my email:-

Kerry Raymond wrote:
Part of the problem of publishing geo-locations (as Paul Blair and I have discussed quite a lot in the past) is in knowing (or guessing) what your users are likely to do with the data.

On our site for each cemetery, we provide the Lat/Long as raw numbers (both DMS and decimal degree formats) and we also provide links to that location in GoogleEarth and GoogleMaps (they are all generated from the same source data so they are always consistent). See here for an example of one of our cemeteries:

As I suspect (but really have no way of knowing) that the most common use people make of this information is via GoogleMaps and then with GE, I tend to favour publishing a geo-location that "looks right" in GM/GE and will tend to "nudge" my GPS waypoint a bit until it appears visually over the cemetery in GM/GE. However, as Paul says, if GM/GE's maps drift a bit, then of course what may start out as "looking right" may end up looking wrong. Sigh!

I suppose I could decouple the raw numbers from the links and treat them as 3 separate waypoints, one which is the true reading I got from the GPS and the other two being nudged to look right in the respective applications (obviously these would still have to be maintained to reflect the "drift" in underlying applications). However, unlike Paul, I have yet to get hate mail from my geo-locations (they whole point of our site being primarily to allow people to just look at the photos online rather than visit the cemetery) so it's not a big issue.

Another option is to configure your GPS receiver to display coordinates in the WGS84 datum. My 1996-vintage Garmin receiver has a very extensive set of datums built in, and I can select which one I want the displayed position to be referred to.

If I want to publish Google Earth compatible coordinates, I can tell my GPS receiver to use the WGS84 datum.

On the other hand, if I'm visiting a cemetery in Britain and I want to provide people with the grid reference on a U.K. Ordnance Survey map, I can tell my GPS receiver to use the Ordnance Survey datum and to display positions as British National Grid coordinates rather than latitude and longitude. There are still a few of us who would rather trust an O.S. map than a satnav system :-)

David Harper
Cambridge, England
Part of the problem of publishing geo-locations - Google Search


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