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Describing datum

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  • steve_galloway
    I would be grateful to anyone who can help me with these points: 1. Is there a gpx convention for describing longitude and lattitude position formats, for
    Message 1 of 9 , Sep 15, 2002
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      I would be grateful to anyone who can help me with these points:

      1. Is there a gpx convention for describing longitude and lattitude position formats, for instance hddd mm ss s, hddd mm mm, British Grid, UTM, etc. Without this field, how does a gpx file describe the type of
      position format of this data?

      2. What is the difference between <ele> and <geoid>?

      3. I don't understand <bounds> - what does a bounding rectangle do?

      Answers on a postage stamp, please!

      Regards,

      Steve Galloway
    • Dan Foster
      Hello, Sunday, September 15, 2002, 12:18:35 PM, Steve wrote: s I would be grateful to anyone who can help me with these points: s 1. Is there a gpx
      Message 2 of 9 , Sep 16, 2002
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        Hello,

        Sunday, September 15, 2002, 12:18:35 PM, Steve wrote:

        s> I would be grateful to anyone who can help me with these points:

        s> 1. Is there a gpx convention for describing longitude and
        s> latitude position formats, for instance hddd mm ss s, hddd mm mm,
        s> British Grid, UTM, etc. Without this field, how does a gpx file
        s> describe the type of position format of this data?

        GPX is a data transfer format, not a display format. GPX always uses
        WGS84 datum, decimal degrees. This ensures that every GPX program
        knows how to interpret the lat/lon values they receive from the GPX
        file.

        GPX-enabled programs can display the GPX data they receive in any
        format they wish. For example, if a GPS Utility user in the UK opened
        a GPX file I saved with EasyGPS, it would display in British Grid
        coordinates (assuming that was the display format set in GPS Utility)
        even though EasyGPS and GPX know nothing about British Grid.

        By the way, this is the way your GPS receiver works - all data is
        stored in WGS84 datum internally, and it gets converted to your
        favorite datum and grid format for display purposes.

        s> 2. What is the difference between <ele> and <geoid>?

        <ele> Elevation - I didn't define this very precisely in the
        documentation. It means what you think it means - the height, in
        meters above mean sea level, of an object.

        <geoidheight> Height, in meters, of WGS-84 earth ellipsoid above mean
        sea level at the point. (This value is useful if you're processing the
        NMEA GGA message)

        Why do we have both? Kjeld (CetusGPS) wanted to be able to express
        all of the data in the standard NMEA messages in GPX. Geoid Height
        was needed for that purpose. For 99% of the applications out there,
        <ele> is all you need. If you care about geoid height, your GPS will
        be happy to tell you the current value, and GPX has a place for you to
        store it.

        s> 3. I don't understand <bounds> - what does a bounding rectangle do?

        <bounds> is an optional tag which specifies the geographic area
        covered by the data in the file. Imagine that someone wrote a search
        engine for GPX files, and you were using it to find all GPX data in
        the UK. If the <bounds> tag was present, the search engine could
        quickly determine if the GPX file overlapped any part of the UK. If
        the <bounds> tag wasn't there, the search engine would have to test
        each waypoint, route point, and track point in the GPX file to see if
        any of them were located inside the UK.

        We added a number of metadata tags (<bounds>, <keywords>, etc) to the
        main <GPX> element to describe the GPX file to search engines,
        websites, and other programs.

        Of course, all of the tags discussed above are optional in GPX, so you
        can ignore them if your application doesn't need them.

        I hope this helps answer some of your questions about GPX!
        --
        Dan Foster
        TopoGrafix - GPS Software, Waypoints, and Maps
        http://www.topografix.com - mailto:egroups@...
      • steve_galloway
        Thanks for your help, appreciated.. SJG
        Message 3 of 9 , Sep 18, 2002
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          Thanks for your help, appreciated.. SJG

          --- In gpsxml@y..., Dan Foster <egroups@t...> wrote:
          > Hello,
          >
          > Sunday, September 15, 2002, 12:18:35 PM, Steve wrote:
          >
          > s> I would be grateful to anyone who can help me with these points:
          >
          > s> 1. Is there a gpx convention for describing longitude and
          > s> latitude position formats, for instance hddd mm ss s, hddd mm mm,
          > s> British Grid, UTM, etc. Without this field, how does a gpx file
          > s> describe the type of position format of this data?
          >
          > GPX is a data transfer format, not a display format. GPX always uses
          > WGS84 datum, decimal degrees. This ensures that every GPX program
          > knows how to interpret the lat/lon values they receive from the GPX
          > file.
          >
          > GPX-enabled programs can display the GPX data they receive in any
          > format they wish. For example, if a GPS Utility user in the UK opened
          > a GPX file I saved with EasyGPS, it would display in British Grid
          > coordinates (assuming that was the display format set in GPS Utility)
          > even though EasyGPS and GPX know nothing about British Grid.
          >
          > By the way, this is the way your GPS receiver works - all data is
          > stored in WGS84 datum internally, and it gets converted to your
          > favorite datum and grid format for display purposes.
          >
          > s> 2. What is the difference between <ele> and <geoid>?
          >
          > <ele> Elevation - I didn't define this very precisely in the
          > documentation. It means what you think it means - the height, in
          > meters above mean sea level, of an object.
          >
          > <geoidheight> Height, in meters, of WGS-84 earth ellipsoid above mean
          > sea level at the point. (This value is useful if you're processing the
          > NMEA GGA message)
          >
          > Why do we have both? Kjeld (CetusGPS) wanted to be able to express
          > all of the data in the standard NMEA messages in GPX. Geoid Height
          > was needed for that purpose. For 99% of the applications out there,
          > <ele> is all you need. If you care about geoid height, your GPS will
          > be happy to tell you the current value, and GPX has a place for you to
          > store it.
          >
          > s> 3. I don't understand <bounds> - what does a bounding rectangle do?
          >
          > <bounds> is an optional tag which specifies the geographic area
          > covered by the data in the file. Imagine that someone wrote a search
          > engine for GPX files, and you were using it to find all GPX data in
          > the UK. If the <bounds> tag was present, the search engine could
          > quickly determine if the GPX file overlapped any part of the UK. If
          > the <bounds> tag wasn't there, the search engine would have to test
          > each waypoint, route point, and track point in the GPX file to see if
          > any of them were located inside the UK.
          >
          > We added a number of metadata tags (<bounds>, <keywords>, etc) to the
          > main <GPX> element to describe the GPX file to search engines,
          > websites, and other programs.
          >
          > Of course, all of the tags discussed above are optional in GPX, so you
          > can ignore them if your application doesn't need them.
          >
          > I hope this helps answer some of your questions about GPX!
          > --
          > Dan Foster
          > TopoGrafix - GPS Software, Waypoints, and Maps
          > http://www.topografix.com - mailto:egroups@t...
        • eeronpoika
          ... Hi, I am responding to an old message, but since it is used as reference through archive, I suppose it is not too late to comment on that. 1) I want to
          Message 4 of 9 , Aug 3, 2003
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            --- In gpsxml@yahoogroups.com, Dan Foster <egroups@t...> wrote:
            > s> 2. What is the difference between <ele> and <geoid>?
            >
            > <ele> Elevation - I didn't define this very precisely in the
            > documentation. It means what you think it means - the height, in
            > meters above mean sea level, of an object.
            >
            > <geoidheight> Height, in meters, of WGS-84 earth ellipsoid above
            > mean sea level at the point. (This value is useful if you're
            > processing the NMEA GGA message)

            Hi,

            I am responding to an old message, but since it is used as reference
            through archive, I suppose it is not too late to comment on that.

            1) I want to check if your definition of <geoidheight> above is what
            you actually meant. I wonder if you wanted to say vice versa, "Height
            of geoid (mean sea level) above WGS84 ellipsoid", not height of
            ellipsoid above geoid?

            2) I would be happy to see height coordinate (height of the point
            above the reference ellipsoid) in GPX-standard - now it is totally
            missing. Neither of <ele> nor <geoidheight> can replace it. The third
            coordinate value in addition to latitude and longitude is height
            above ellipsoid. The physical heights are actually properties of a
            point defined by (lat,lon,height above ellipsoid). To be accurate,
            physical heights are vector-functions of (lat,lon,height above
            ellipsoid), even though only the scalar part is normally used. Summa
            summarum: height above reference ellipsoid canät be substituted by
            other heights.

            With Kind Regards,
            Eino Uikkanen
            www.iki.fi/eino.uikkanen/gb/
          • Dan Foster
            Hello, Sunday, August 3, 2003, 4:13:17 PM, Eino wrote: ... e Hi, e I am responding to an old message, but since it is used as reference e through archive, I
            Message 5 of 9 , Aug 4, 2003
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              Hello,

              Sunday, August 3, 2003, 4:13:17 PM, Eino wrote:

              e> --- In gpsxml@yahoogroups.com, Dan Foster <egroups@t...> wrote:
              >> s> 2. What is the difference between <ele> and <geoid>?
              >>
              >> <ele> Elevation - I didn't define this very precisely in the
              >> documentation. It means what you think it means - the height, in
              >> meters above mean sea level, of an object.
              >>
              >> <geoidheight> Height, in meters, of WGS-84 earth ellipsoid above
              >> mean sea level at the point. (This value is useful if you're
              >> processing the NMEA GGA message)

              e> Hi,

              e> I am responding to an old message, but since it is used as reference
              e> through archive, I suppose it is not too late to comment on that.

              e> 1) I want to check if your definition of <geoidheight> above is what
              e> you actually meant. I wonder if you wanted to say vice versa, "Height
              e> of geoid (mean sea level) above WGS84 ellipsoid", not height of
              e> ellipsoid above geoid?

              I copied this definition word for word from the description of the
              NMEA GGA sentence in my Magellan 315 user's manual. But you are
              correct, I believe the name we chose is confusing. It probably should
              be called <ellipsoidheight>, since it's the height of the ellipsoid
              above the geoid (mean sea level).

              e> 2) I would be happy to see height coordinate (height of the point
              e> above the reference ellipsoid) in GPX-standard - now it is totally
              e> missing. Neither of <ele> nor <geoidheight> can replace it. The third
              e> coordinate value in addition to latitude and longitude is height
              e> above ellipsoid. The physical heights are actually properties of a
              e> point defined by (lat,lon,height above ellipsoid). To be accurate,
              e> physical heights are vector-functions of (lat,lon,height above
              e> ellipsoid), even though only the scalar part is normally used. Summa
              e> summarum: height above reference ellipsoid canät be substituted by
              e> other heights.

              Perhaps I'm not understanding you correctly. It seems to me that your
              new height "h" is just the difference between <ele> and <geoidheight>.

              h = height above WGS84 ellipsoid
              ele = height above mean sea level (geoid)
              geoidheight = height of WGS84 ellipsoid above mean sea level (geoid)

              so, h = ele - geoidheight
              --

              Dan Foster
              TopoGrafix - GPS Software, Waypoints, and Maps
              http://www.topografix.com - mailto:egroups@...
            • eeronpoika
              ... reference ... what ... versa, Height ... should ... point ... totally ... third ... a ... accurate, ... Summa ... by ... your ... . ... I
              Message 6 of 9 , Aug 4, 2003
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                --- In gpsxml@yahoogroups.com, Dan Foster <egroups@t...> wrote:
                > Hello,
                >
                > Sunday, August 3, 2003, 4:13:17 PM, Eino wrote:
                >
                > e> --- In gpsxml@yahoogroups.com, Dan Foster <egroups@t...> wrote:
                > >> s> 2. What is the difference between <ele> and <geoid>?
                > >>
                > >> <ele> Elevation - I didn't define this very precisely in the
                > >> documentation. It means what you think it means - the height, in
                > >> meters above mean sea level, of an object.
                > >>
                > >> <geoidheight> Height, in meters, of WGS-84 earth ellipsoid above
                > >> mean sea level at the point. (This value is useful if you're
                > >> processing the NMEA GGA message)
                >
                > e> Hi,
                >
                > e> I am responding to an old message, but since it is used as
                reference
                > e> through archive, I suppose it is not too late to comment on that.
                >
                > e> 1) I want to check if your definition of <geoidheight> above is
                what
                > e> you actually meant. I wonder if you wanted to say vice
                versa, "Height
                > e> of geoid (mean sea level) above WGS84 ellipsoid", not height of
                > e> ellipsoid above geoid?
                >
                > I copied this definition word for word from the description of the
                > NMEA GGA sentence in my Magellan 315 user's manual. But you are
                > correct, I believe the name we chose is confusing. It probably
                should
                > be called <ellipsoidheight>, since it's the height of the ellipsoid
                > above the geoid (mean sea level).
                >
                > e> 2) I would be happy to see height coordinate (height of the
                point
                > e> above the reference ellipsoid) in GPX-standard - now it is
                totally
                > e> missing. Neither of <ele> nor <geoidheight> can replace it. The
                third
                > e> coordinate value in addition to latitude and longitude is height
                > e> above ellipsoid. The physical heights are actually properties of
                a
                > e> point defined by (lat,lon,height above ellipsoid). To be
                accurate,
                > e> physical heights are vector-functions of (lat,lon,height above
                > e> ellipsoid), even though only the scalar part is normally used.
                Summa
                > e> summarum: height above reference ellipsoid canät be substituted
                by
                > e> other heights.
                >
                > Perhaps I'm not understanding you correctly. It seems to me that
                your
                > new height "h" is just the difference between <ele> and
                <geoidheight>.
                >
                > h = height above WGS84 ellipsoid
                > ele = height above mean sea level (geoid)
                > geoidheight = height of WGS84 ellipsoid above mean sea level (geoid)
                >
                > so, h = ele - geoidheight
                > --
                >
                > Dan Foster
                > TopoGrafix - GPS Software, Waypoints, and Maps
                > http://www.topografix.com - mailto:egroups@t...

                I want to check once more - actually I did also take a direct
                quotation from NMEA0183-GGA-message description and it was "Height of
                geoid (mean sea level) above WGS84 ellipsoid". Now our quotations of
                GGA disagree?! According to my interpretation h = ele + geoidheight
                (sum). I checked the quatation once more - could you pls do the same,
                but from NMEA source, not from Magellans ;-). I understood, that you
                anyway meant it to be the same as in GGA?

                This "my new h" is not just another height - it is the one and only
                height used in coordinate calculations, e.g. when we convert 3D-
                rectangular coordinates (X,Y,Z) to geodetic coordinates (lat,lon,h).
                Therefore it is very important - actually mandatory in geodetic
                applications. My dream of a point element would then be e.g.

                <wpt lat="99.999" lon="99.999" height="9999.99">
                <ele>999.999</ele>
                <geoidheight>9999.99</geoidheight>
                etc.

                This is because "my h" is _coordinate_ value as are lat and lon -
                other heights are _properties_ of the point (lat,lon,h).

                With Kind Regards,
                Eino Uikkanen
                www.iki.fi/eino.uikkanen/gb/
              • Dan Foster
                Hello, Monday, August 4, 2003, 11:43:39 AM, Eino wrote: e I want to check once more - actually I did also take a direct e quotation from NMEA0183-GGA-message
                Message 7 of 9 , Aug 4, 2003
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                  Hello,

                  Monday, August 4, 2003, 11:43:39 AM, Eino wrote:

                  e> I want to check once more - actually I did also take a direct
                  e> quotation from NMEA0183-GGA-message description and it was "Height of
                  e> geoid (mean sea level) above WGS84 ellipsoid". Now our quotations of
                  e> GGA disagree?! According to my interpretation h = ele + geoidheight
                  e> (sum). I checked the quatation once more - could you pls do the same,
                  e> but from NMEA source, not from Magellans ;-). I understood, that you
                  e> anyway meant it to be the same as in GGA?

                  We intended it to be the same as GGA. I went back to Kjeld's original
                  request to include it. He defined it as you do:
                  <geoidheight> Height of geoid (mean sea level) above WGS84 ellipsoid

                  I checked the description in Magellan's manual. They write:
                  Geoidal separation - difference between the WGS-84 earth ellipsoid and
                  mean sea level (geoid), "-" = mean sea level below ellipsoid.

                  That description matches your definition as well.

                  I must have switched the two terms when writing the documentation.
                  I'll change the documentation tomorrow, unless someone objects, to the
                  following:
                  <geoidheight> Height of geoid (mean sea level) above WGS84 ellipsoid

                  As far as I know, Kjeld's CetusGPS is the only program out there using
                  <geoidheight> now, and based on his original message, I assume he used
                  the correct definition rather than my mistaken one.

                  e> This "my new h" is not just another height - it is the one and only
                  e> height used in coordinate calculations, e.g. when we convert 3D-
                  e> rectangular coordinates (X,Y,Z) to geodetic coordinates (lat,lon,h).
                  e> Therefore it is very important - actually mandatory in geodetic
                  e> applications. My dream of a point element would then be e.g.

                  e> <wpt lat="99.999" lon="99.999" height="9999.99">
                  e> <ele>999.999</ele>
                  e> <geoidheight>9999.99</geoidheight>
                  e> etc.

                  e> This is because "my h" is _coordinate_ value as are lat and lon -
                  e> other heights are _properties_ of the point (lat,lon,h).

                  I realize that you need the height above the ellipsoid to do precise
                  calculations. In instances where you have both <ele> and
                  <geoidheight>, you can calculate this:
                  h = height above WGS84 ellipsoid
                  ele = height above mean sea level (geoid)
                  geoidheight = Height of geoid (mean sea level) above WGS84 ellipsoid

                  so, h = ele + geoidheight

                  And if you have <ele> but not <geoidheight>, you're out of luck. You
                  won't be able to do your calculations with the correct height data,
                  unless you've got your own table of geoidheight values for the Earth.

                  I did a quick search on the newsgroups for "garmin altitude geoid",
                  and the results are troubling. Nobody has a clear answer to whether
                  the altitude sent with Garmin waypoints is measured relative to the
                  geoid, or to the WGS-84 ellipsoid. I've always assumed it was
                  relative to the geoid (since end users think of altitude relative to
                  mean sea level). I'd run outside and do a test, but it's pouring rain
                  here...

                  --
                  Dan Foster
                  TopoGrafix - GPS Software, Waypoints, and Maps
                  http://www.topografix.com - mailto:egroups@...
                • eeronpoika
                  ... Fine – now we and all the definitions do agree and no harm was done :-) ... Make it two. My GeoConv also reads and writes in addition to
                  Message 8 of 9 , Aug 4, 2003
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                    --- In gpsxml@yahoogroups.com, Dan Foster <egroups@t...> wrote:
                    > We intended it to be the same as GGA. I went back to Kjeld's
                    > original request to include it. He defined it as you do:
                    > <geoidheight> Height of geoid (mean sea level) above WGS84
                    > ellipsoid
                    >
                    > I checked the description in Magellan's manual. They write:
                    > Geoidal separation - difference between the WGS-84 earth ellipsoid
                    > and mean sea level (geoid), "-" = mean sea level below ellipsoid.
                    >
                    > That description matches your definition as well.
                    >
                    > I must have switched the two terms when writing the documentation.
                    > I'll change the documentation tomorrow, unless someone objects, to
                    > the following:
                    > <geoidheight> Height of geoid (mean sea level) above WGS84 ellipsoid

                    Fine – now we and all the definitions do agree and
                    no harm was done :-)

                    > As far as I know, Kjeld's CetusGPS is the only program out there
                    > using <geoidheight> now, and based on his original message, I
                    > assume he used the correct definition rather than my mistaken one.

                    Make it two. My GeoConv also reads and writes <geoidheight> in
                    addition to <ele> and calculates h = <ele> + <geoidheight>.
                    If you want to have a look, you find GeoConv from address
                    www.iki.fi/eino.uikkanen/geoconvgb/

                    Wbr,
                    Eino
                    www.iki.fi/eino.uikkanen/gb/
                  • Kjeld Jensen
                    ... I agree to this definition. It defines what I was thinking about, when we discussed it :-) I recall that we ended up including all data from the RMC and
                    Message 9 of 9 , Aug 7, 2003
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                      >I'll change the documentation tomorrow, unless someone objects, to the
                      >following:
                      ><geoidheight> Height of geoid (mean sea level) above WGS84 ellipsoid

                      I agree to this definition. It defines what I was thinking about,
                      when we discussed it :-)

                      I recall that we ended up including all data from the RMC and GGA
                      sentences in GPX, and based on this, the definition above is also
                      correct.

                      Sorry for being passive in this and other discussions on the list,
                      but I am quite busy at the moment.

                      Kjeld
                      --
                      ________________________
                      Kjeld Jensen
                      N 55°22' E 10°24'
                      Email: kjeld@...
                      http://www.CetusGPS.dk
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