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I asked a friend of mine who is a professional cartographer if he could explain these issues and this is what he said : "the latitude and longitude of a place depend upon what datum and ellipsoid is being used (the two are often mentioned in one as, for example, OSGB36). The ellipsoid is the theoretical shape of the earth, and there are a number as some are better fitted to certain parts of the globe than others. The datum is the point or points at which this ellipsoid is actually ‘fixed' to the body of the Earth. The OS datum and ellipsoid are referred to as OSGB36, whilst GPS use WGS84. The two will forever give a different lat and long, although not by much, and the OS are considering putting the lat and long conforming to WGS84 on their paper maps . The third factor, other than datum and ellipsoid, needed to put the surface of the Earth onto a two dimensional plane is the projection, but this will not alter the lat and long readings" Seems clear to me. David On 5 December 2014 at 09:34, Anne Taylor <[log in to unmask]> wrote: > Dear all > > Don't know if these tools might help: > > http://www.gridreferencefinder.com/ > > http://www.fieldenmaps.info/cconv/ > > (These tools and others are listed on our web pages at > http://www.lib.cam.ac.uk/deptserv/maps/links.html#scales) > > Anne > > > > On 05/12/2014 09:13, Jane Stevenson wrote: > > Hi there, > > Many thanks to everyone who’s responded, it’s really helpful. It sounds like we can provide the ability to add coordinates, and assume WGS 84, but it would be good to also allow cataloguers to use OS national grid coordinates and enable conversion, so that we can just store lat/long in the EAD. > > cheers, > Jane > > > > > On 4 Dec 2014, at 18:31, humphrey <[log in to unmask]> <[log in to unmask]> wrote: > > > Yes, I am lurking. > > Yes, I think you can safely assume that anyone supplying latitude and longitude as a pair of decimal numbers is using WGS-84. It is pretty much a standard now, used well beyond GPS, and anyone with actual latitude and longitude in a different format will probably have the sub-degree information as minutes and seconds, which would need a whole other form to input. > > However, I also think that enabling the interface to also accept Ordnance Survey national grid coordinates is important. These are not latitude and longitude at all, but numbers of kilometres east and north from a point south west of Lands End (“grid origin”), and only have meaning within Britain. > > Note that any spatially-enabled database system (we use Postgres) includes functions to convert between coordinate systems, and these usually use the numeric IDs for coordinate systems defined by the EPSG Geodetic Parameter Registry (www.epsg-registry.org). In this, the British National Grid, or "OSGB 1936 “ has SRID 27700, the “European Terrestrial Reference System 1989” is 3034, and WGS-84 is 4326. A UI where there were boxes to type in a pair of coordinates, and a drop-down menu to select the coordinate system, would work so long as you had that conversion functionality. I would store everything as WGS-84. > > If anyone has found the maps in our Vision of Britain site a little odd-looking, it is because it is built around ETRS-89. > > … and I will now ask my own obscure question …. > > Humphrey > > > On 4 Dec 2014, at 17:38, Jane Stevenson <[log in to unmask]> <[log in to unmask]> wrote: > > Hi there, > > A rather specialised question, but geographers may lurk on this list….(Humphrey…are you there?) > > I’m looking at enabling Archives Hub contributors to add geographical coordinates to the geographical names they enter. But apparently latitude and longitude are not as straightforward as I thought. > > There are a number of systems that can be used, and the system you use can affect the entry. > > Wikipedia: "Latitude and longitude values can be based on different geodetic systems or datums, the most common being WGS 84, a global datum used by all GPS equipment. Other datums are significant because they were chosen by a national cartographical organisation as the best method for representing their region, and these are the datums used on printed maps. The latitude and longitude on a map may not be the same as on a GPS receiver. Coordinates from the mapping system can sometimes be roughly changed into another datum using a simple translation. For example, to convert from ETRF89 (GPS) to the Irish Grid add 49 metres to the east, and subtract 23.4 metres from the north.” > > We can hardly expect archivists to know whether they are using WGS84 or OSGB36 (which the OS use) or something else. But this seems to mean that if you put Lat: 54.428736 Long: -2.961333 it could in theory refer to a slightly different point depending on the system you are using. There is not a universally agreed and implemented system. Would it be terribly bad for us to assume WGS84? Should we just avoid adding anything about the system used? WGS84 is what is used by GIS systems apparently, so I assume online lookups such as http://www.latlong.net/ use it, but archivists may well refer to OS maps for their lat/long. > > cheers, > Jane > > Jane Stevenson > The Archives Hub > T 0161 275 6055 > W archiveshub.ac.uk > Skype janestevenson > Twitter @archiveshub, @janestevenson > J Floor, Sackville Street Building, Sackville Street, Manchester, M1 3BB > jisc.ac.uk > Jisc is a registered charity (number 1149740) and a company limited by guarantee which is registered in England under Company No. 5747339, Jisc’s registered office is: One Castlepark, Tower Hill, Bristol, BS2 0JA. T 0203 697 5800 > > Contact the list owner for assistance at [log in to unmask] > > For information about joining, leaving and suspending mail (eg during a holiday) see the list website athttps://www.jiscmail.ac.uk/cgi-bin/webadmin?A0=archives-nra > > Humphrey Southall > Professor of Historical Geography/ > Director, GB Historical GIS > University of Portsmouth > Geography Dept, Buckingham Bldg, > Lion Terrace, Portsmouth PO1 3HE, [log in to unmask] > > Contact the list owner for assistance at [log in to unmask] > > For information about joining, leaving and suspending mail (eg during a holiday) see the list website athttps://www.jiscmail.ac.uk/cgi-bin/webadmin?A0=archives-nra > > Contact the list owner for assistance at [log in to unmask] > > For information about joining, leaving and suspending mail (eg during a holiday) see the list website athttps://www.jiscmail.ac.uk/cgi-bin/webadmin?A0=archives-nra > > > -- > Anne Taylor > Head of Map Department, Cambridge University Library, West Road, Cambridge CB3 9DR > Tel: 01223-333041. Fax: 01223-333160. email: [log in to unmask]://www.lib.cam.ac.uk/deptserv/maps/ > > !!VAT NUMBER: G.B. 823 8476 09!! > > Contact the list owner for assistance at > [log in to unmask] > > For information about joining, leaving and suspending mail (eg during a > holiday) see the list website at > https://www.jiscmail.ac.uk/cgi-bin/webadmin?A0=archives-nra > -- David Contact the list owner for assistance at [log in to unmask] For information about joining, leaving and suspending mail (eg during a holiday) see the list website at https://www.jiscmail.ac.uk/cgi-bin/webadmin?A0=archives-nra