Re: [dhcwg] Fwd: Working Group Last Call: Location Configuration Information for GEOPRIV
John Schnizlein <jschnizl@cisco.com> Wed, 09 July 2003 16:32 UTC
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Date: Wed, 09 Jul 2003 12:31:21 -0400
To: Greg Troxel <gdt@ir.bbn.com>
From: John Schnizlein <jschnizl@cisco.com>
Subject: Re: [dhcwg] Fwd: Working Group Last Call: Location Configuration Information for GEOPRIV
Cc: dhcwg@ietf.org, geopriv@mail.apps.ietf.org
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References: <000501c340ab$46784be0$220d0d0a@mlinsnerzk7abh> <000501c340ab$46784be0$220d0d0a@mlinsnerzk7abh>
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At 10:43 AM 7/9/2003, Greg Troxel wrote: >But almost no one actually understands what altitude means (I didn't >until I read several geodesy textbooks)... My point is that referring to >altitude/height other than with a specific reference to an established >vertical or 3-dimensional datum is madness. It seems we don't either. Thank you for the tutorial background. Is your recommendation that we specify no default? >For location in the US (sorry for being US-centric; I'm not >familiar enough with datums in the rest of the world to comment >precisely), we have multiple possible meanings of altitude: > relative to MLLW (nautical charts) > relative to NGVD29 (older topo maps, typically NAD27 horizontal datum) > relative to NAVD88 (newer topo maps, typically NAD83 horizontal datum) > relative to WGS84 ellipsoidal height (not used for civil heights) > relative to WGS84 geoid (should match NAVD88 closely enough for > "mapping purposes") > >The latter is what a typical GPS receiver produces. GPS receivers >calculate positions on the ellipsoid (obtaining an ellipsoidal height) >and apply a geoid model to obtain an estimate of orthometric height. > >This is all overkill for geopriv, but I'm really uncomfortable with a >notion that altitude is in a local convention for datums that don't >define altitude. Even this is ambiguous: the North American Datum of >1983 is a horizontal datum, but "everybody knows" that orthometric >heights given in conjunction with this datum are in NAVD88. Except on >nautical charts. > > From: "James M. Polk" <jmpolk@cisco.com> > Date: Wed, 09 Jul 2003 00:13:21 -0500 > > We state that if the datum doesn't define a "0" altitude, we define it > as "mean low tide". Does this not cover that angle? This seems > pointless when the altitude is measured in floors, but I could be > wrong. > >Altitude should always have a datum. Using orthometric height >relative to the WGS84 geoid is a fine default choice. Mean low water >only makes sense at the shore; WGS84 is defined globally. > >There should be two datums: horizontal and vertical. For datums which >are 3-dimensional, like WGS84, the same code can be used in both >slots. Either that, or datum codepoints should be required to be >either a 3-D datum or a pair of horizontal/vertical datums >(e.g. NAD83/NAVD88). The use of pairs is really quite natural; >vertical and horizontal datums were typically developed alongside each >other. (This must be defined already in other standards, but the "if >no vertical datum then use mean low water" makes me think not quite >enough of another representation was adopted.) Are you recommending that the default for altitude be WGS84 if not otherwise specified by the datum value in the option? >Perhaps we should define a 'floor' datum. Floors aren't really a >"unit of measurement", since they aren't necessarily the same size, 13 >might not exist, etc. The floor datum is simple: the value is a >number (no units), and corresponds to elevator labeling if there are >elevators, or posted labels if those exist. If not, 1 is where people >normally enter, and whole/half flights of stairs change value by >1/0.5. Floor levels are very useful, perhaps more so than orthometric >height in buildings, but we shouldn't try to make them into an >altitude, since they simply don't have the same properties. And every >building is different; a floor number without an identified building >doesn't make sense. It was not our intent to make floors into altitudes. Since your tutorial demonstrated that most of us do not understand "datum", this term is probably not better than "unit of measurement". There is no requirement that all units of measurement have the same scale properties; some might be only nominal or ordinal measurement units (like hardness), but others might be interval (ratios of differences meaningful, like degrees of temperature) and others might have meaningful ratio measurement properties. We need not get into the depths (pun intended) of measurement theory for this term. >... >I haven't been following very closely; sorry if I'm off in space or >seeming difficult. I think that trying to follow established geodetic >practice will make some things smoother. For those people that don't >know their altitude, it won't matter anyway. We are happy to accept specific text that makes this document better "follow established geodetic practice". Is it just the default for altitude for which you want to recommend text? John _______________________________________________ dhcwg mailing list dhcwg@ietf.org https://www1.ietf.org/mailman/listinfo/dhcwg
- [dhcwg] Fwd: Working Group Last Call: Location Co… Ralph Droms
- Re: [dhcwg] Fwd: Working Group Last Call: Locatio… Robert Elz
- Re: [dhcwg] Fwd: Working Group Last Call: Locatio… John Schnizlein
- Re: [dhcwg] Fwd: Working Group Last Call: Locatio… Robert Elz
- Re: [dhcwg] Fwd: Working Group Last Call: Locatio… Andrew Daviel
- RE: [dhcwg] Fwd: Working Group Last Call: Locatio… Marc Linsner
- Re: [dhcwg] Fwd: Working Group Last Call: Locatio… Henning Schulzrinne
- Re: [dhcwg] Fwd: Working Group Last Call: Locatio… creediii
- [dhcwg] Unit of Measurement...? James M. Polk
- Re: [dhcwg] Fwd: Working Group Last Call: Locatio… John Schnizlein
- [dhcwg] Re: Unit of Measurement...? James M. Polk
- Re: [dhcwg] Fwd: Working Group Last Call: Locatio… James M. Polk
- RE: [dhcwg] Fwd: Working Group Last Call: Locatio… Marc Linsner
- Re: [dhcwg] Fwd: Working Group Last Call: Locatio… Henning Schulzrinne
- Re: [dhcwg] Fwd: Working Group Last Call: Locatio… Greg Troxel
- [dhcwg] Re: Unit of Measurement...? Henning Schulzrinne
- [dhcwg] Re: Unit of Measurement...? Greg Troxel
- Re: [dhcwg] Fwd: Working Group Last Call: Locatio… Greg Troxel
- Re: [dhcwg] Fwd: Working Group Last Call: Locatio… Greg Troxel
- [dhcwg] Re: Unit of Measurement...? Carl Reed
- Re: [dhcwg] Fwd: Working Group Last Call: Locatio… Dominic Pinto
- Re: [dhcwg] Fwd: Working Group Last Call: Locatio… Carl Reed