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Re: [Geojson] vertical datum definition

Martin Daly <martin.daly@cadcorp.com> Thu, 20 October 2022 08:47 UTC

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From: Martin Daly <martin.daly@cadcorp.com>
To: "Hedley, Mark" <mark.hedley=40metoffice.gov.uk@dmarc.ietf.org>, Howard Butler <howard@hobu.co>
CC: "geojson@ietf.org" <geojson@ietf.org>
Thread-Topic: [Geojson] vertical datum definition
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Date: Thu, 20 Oct 2022 08:47:22 +0000
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Subject: Re: [Geojson] vertical datum definition
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Hi Mark,

> My restated question is:
> Given (1), that a data provider has encoded heights in the GeoJSON payload I
> am loading, how do I deal with the interpretation / assumption that the data
> provider has provided heights / altitudes with respect to local mean sea level,
> but the specification mandates that I interpret these with respect to the
> WGS84 vertical datum (with significant offset value implicastions in many
> cases)?
> 
> Height / altitude with respect to the WGS84 datum is rarely used and often
> misunderstood, but is the only way to interpret GeoJSON payloads given the
> specification.
> 
> Is this question more clear than my previous attempt?

Section 4 contains these weasel words, that allow you and your data supplier to agree interpretations / assumptions in advance, or to use a CRS other than WGS84 Lon/Lat:

"However, where all involved parties have a prior arrangement, alternative coordinate reference systems can be used without risk of data being misinterpreted."

https://www.rfc-editor.org/rfc/rfc7946#section-4

Regards,
Martin

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