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Subject: Re: [emergency] RE: Notes form last GIS SC meeting


On Mar 22, 2004, at 4:39 PM, Carl Reed wrote:

> At the last GIS SC meeting we discussed the issue of whether WGS 84 - 
> Lat/Long is adequate for this go around of the CAP specification. The 
> issue was raised with regard to using USNG-NAD83. The unanimous 
> consensus of the group was that USNG is for use at the user interface 
> level. Therefore, for a message protocol, such as CAP WGS 84 - 
> Lat/Long is adequate and CAP spec should remain the  way it is. 
> Further, by leaving CAP with WGS 84 - Lat/Long, the CAP spec remains 
> more international in nature and not restricted to the US market 
> space.
>  
> Also, by way of clarification, Dave Danko (ESRI) added:
>  NAD 83 vs WGS 84: Technically these are the same for mapping, 
> charting, and navigation purposes. Geodetically, of course they are 
> different but I think they are the same for our purposes. To keep this 
> standard international we should use WGS 84. Perhaps in a users guide 
> for the US it could be mentioned that NAD 83 coordinates work fine 
> here and that NAD 83 coordinates could be used. However (Carl's note), 
> this would mean that we would need some code to allow the receiving 
> application to know that the coordinates are NAD 83 and not WGS 
> Lat/Long.
> As we are working at an accuracy level of many feet/meters, the 
> geodetic differences are not important.

While I am certainly no geocoding expert, would Dave's comments not 
imply that the "best" way to handle this is to "facilitate the use of 
various geocoding standards through the use of a coding identifier 
(e.g.: specified in an attribute)"? What we have make work for us, but 
that is not the question. Does it work for the target implementers is 
the question. Saying that it is "adequate" gives to much of a feeling 
like that is what we ended up with because we didn't take the time to 
do it right.

> We then went on to discuss the issue of "altitude" and "ceiling" notes 
> section. We think the words "per the WGS 84 datum" should be 
> removed. We think that this phrase will add communication confusion, 
> especially for the geographically literate.

I would assume this also means reworking the last sentence of section 
1.3.4 (<area>) as well, correct?

> The WGS ellipsoid is a best fit around the world and differs from MSL 
> by varied amounts around the world. So does one put a value 
> here, including/excluding the delta from the WGS-84 
> ellipsoid, a height referenced to the ellipsoid? What is the meaning 
> of the value here? I think we should just have it from (local) MSL for 
> the area of interest. (With ellipsoid heights there is a danger of 
> having to provide negative values for areas above sea level). This 
> would be plenty specific for the purposes intended for this standard. 
> Perhaps this should be corrected in a future version of the standard.

You lost me here in geo-lingo. What, specifically, does this paragraph 
pertain to (section in spec, what would a rough first stab at the 
proposed language look like, and how will that language address the 
section of concern?

Allen

--
R. Allen Wyke
Chair, OASIS Emergency Management TC
emergency-tc@earthlink.net



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