source: trunk/README.GMAPS @ 1032

Last change on this file since 1032 was 960, checked in by Dominic Hargreaves, 15 years ago

Fix documentation re British grid stuff.

File size: 2.3 KB
Line 
1As of 0.52, OpenGuides supports Google Maps using the API. The support has a
2few rough edges that still need ironing out, mostly relating to ellipsoids,
3and for that reason they aren't exposed by default.
4
5To use Google Maps, you need to get an API key from
6http://www.google.com/apis/maps/signup.html. When you've put this value into
7the config file as gmaps_api_key, the maps become available to you in two
8places. Firstly, if you've set "show_gmap_in_node_display", and if the user
9has set their preferences to display Google Maps, any node which has
10location information will get an inset map pinpointing the location.
11Secondly, the URL
12http://.../wiki.cgi?action=index;format=map becomes a large map with a list
13of all nodes (regardless of the user settings). The latter will become very
14slow on any sizable guide (there is work being done on speeding up indexes
15in general - please see the mailing list). If you don't get maps displayed
16when you know that the node has location information, read on.
17
18Ellipsoids and map accuracy
19---------------------------
20
21If you are based in the US and inputting points using latitude/longitude,
22the values in your database will almost certainly correspond to points on
23Google Maps. If so, simply change the "ellipsoid" configuration variable
24to "WGS-84" if it's not already set to this, and your maps will display.
25
26If you are using the British Grid system things are more complicated.
27To display accurate maps, you need to have the optional perl module
28Geo::HelmertTransform installed (libgeo-helmerttransform-perl in Debian).
29If this isn't possible, you can still get Google Maps to display by setting
30the configuration variable force_wgs84 to 1, but points will be offset from
31where they should be, normally by about 50 metres. This is because the
32British Grid uses a different ellipsoid from Google Maps. This is not
33recommended; if you do so, you must make users aware of the flaw and tell
34them not to try and "correct" location data!
35
36You may be unlucky enough to be using the UTM input modes (ie not British
37or Irish National Grid) but have data entered in a mixture of formats.
38This is likely if your users are from the UK, as you may find a mixture of
39WGS-84 and Airy (ie British Grid style) based nodes. In this case, you
40*do* need to correct any that are offset. Only do this if your users
41input Lat/long values rather than grid Eastings/Northings.
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