Re: [emergency-gis] WGS 84 Comments on CAP

[Eliot Christian <echristi@usgs.gov>]

At 03:00 PM 6/26/2003 +0000, Bill Schroeder <bschroeder@esri.com> wrote:

>Most of the world uses meters, however the aeronautical navigation people 
>use feet. So if most of the CAP messages where height is needed are 
>related to flight warnings or restrictions then it should be feet.
>
>Most of the US States use feet - however some use the international survey 
>foot vs the US foot. Some of them have agreed that when they provide data 
>to USGS it will be in meters. So it's all screwed up.
>
>I think we should pick one - and demand all messages use it. Probably ft.

OK, we're just trying to fit into this world, not reform it :-)

>I think we let people provide what ever level of precision they want.
>
>If the polygon in the message covers an area that can be drawn on a 
>1:250,000 scale map - only single precision is needed.
>If the message is trying to precisely describe several city blocks then 
>you need double-precision
>
>We would need to know exactly how it's to be used before we could say one 
>or the other.
>
>In a majority of cases double precision is overkill and causes the 
>coordinate strings to be voluminous.

Your point is valid but I am thinking of a different aspect.
How about adding the following language to CAP's "WGS84 Note"?

   Also, developers of geographic facilities should exercise
   caution in the alignment of "precision" to "accuracy". For
   example, consider the possible accuracy of a world map
   comprising 360 degrees longitude by 180 degrees latitude.
   When displayed at 400 pixels wide by 200 pixels, the accuracy
   per pixel can be no more than a single unit of longitude
   and latitude. The precision of the WGS-84 values in the
   example should therefore be whole degrees, not some number
   of decimal places.

Eliot