Hi Phil, Yes, I think strictly this is correct. But it does mean that, for data that do not specify a datum, the only legitimate way to plot such data as a map is as a latitude-longitude plot. Projection to any other coordinate system would require knowledge of the datum in order to do the mathematical transformation accurately (I think... possibly there are some exceptions).
The suggestion for applications to print "health warnings" for underspecified data is a good one. Maybe future versions of CF should make the datum specification mandatory, where it is known? Jon -----Original Message----- From: Bentley, Philip [mailto:[email protected]] Sent: 19 April 2010 16:52 To: Jonathan Blower; Mike Grant Cc: [email protected] Subject: RE: [CF-metadata] Default datum for latitude and longitude? Hi Jon, Indeed, the UM global climate model, for example, assumes a spherical earth. In the absence of specific datum information I think I would probably caution against assuming any particular datum is applicable. I can understand the temptation to use WGS 1984 as a default because it is widely known and used. However, this implies a degree of accuracy in the coordinate data which in many instances may well not be justified - indeed its use might even compound any relative location errors. As you rightly suggest, if the end-use/application demands a particular spatial accuracy then any datasets which demonstrably do not not meet those requirements should probably not be included in the analysis - or else should be with appropriate caveats, provisos and disclaimers! Regards, Phil > -----Original Message----- > From: [email protected] > [mailto:[email protected]] On Behalf Of Jonathan Blower > Sent: 19 April 2010 16:23 > To: Mike Grant > Cc: [email protected] > Subject: Re: [CF-metadata] Default datum for latitude and longitude? > > Hi Mike, > > Yes, that's what I've been doing so far, but I seem to > remember someone telling me that models tend to be formulated > on a spherical earth. But maybe if the precise geolocation > of the model grid points is important (e.g. for small-scale > models) the datum will always be specified, or a different > projection used. > > Cheers, Jon > > -----Original Message----- > From: Mike Grant [mailto:[email protected]] > Sent: 19 April 2010 16:07 > To: Jonathan Blower > Cc: [email protected] > Subject: Re: [CF-metadata] Default datum for latitude and longitude? > > On 19/04/10 15:43, Jonathan Blower wrote: > > specification, which datum should be assumed? Spherical Earth? > WGS84? > > If you're picking one at random, I'd go for WGS84 - that's a > pretty safe bet for a lot of remote sensed and GPS related data. > > Cheers, > > Mike. > > -------------------------------------------------------------- > ---------- > -------- > Plymouth Marine Laboratory > > Registered Office: > Prospect Place > The Hoe > Plymouth PL1 3DH > > Website: www.pml.ac.uk > Registered Charity No. 1091222 > PML is a company limited by guarantee > registered in England & Wales > company number 4178503 > > PML is a member of the Plymouth Marine Sciences Partnership > Website: www.pmsp.org.uk > -------------------------------------------------------------- > ---------- > -------- > This e-mail, its content and any file attachments are confidential. > > If you have received this e-mail in error please do not copy, > disclose it to any third party or use the contents or > attachments in any way. > Please notify the sender by replying to this e-mail or e-mail > [email protected] and then delete the email without making > any copies or using it in any other way. > > The content of this message may contain personal views which > are not the views of Plymouth Marine Laboratory unless > specifically stated. > > You are reminded that e-mail communications are not secure > and may contain viruses. Plymouth Marine Laboratory accepts > no liability for any loss or damage which may be caused by viruses. > -------------------------------------------------------------- > ---------- > -------- > _______________________________________________ > CF-metadata mailing list > [email protected] > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata > _______________________________________________ CF-metadata mailing list [email protected] http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
