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- MapSource-Elevation-Question
- 08-11-2010
  Re: MapSource Elevation Question
| Bert Hyman | 08-11-2010 |
   Re: MapSource Elevation Question
| Left-is-Gauche | 08-11-2010 |
 Re: MapSource Elevation Question
| Gene E. Bloch | 08-11-2010 |
 Re: MapSource Elevation Question
| Gene E. Bloch | 08-11-2010 |
 Re: MapSource Elevation Question
| Gene E. Bloch | 08-18-2010 |
| Re: MapSource Elevation Question
| Gene E. Bloch | 08-18-2010 |
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On Thu, 12 Aug 2010 08:45:47 +0100, Mike Lane wrote:
>> And where would the origin of coordinates be? What could be more logical
>> than the center of the earth? And this is the germ of my remark about
>> altitude from the center of the earth. And what would my altitude be? If I
>> were flying a plane, it would be my rho minus the value of rho *at the
>> ground* at my lat/lon.
>>
>> Now on to driving over a pass... The altitude I want to see is the altitude
>> above sea level, or rho at the car minus rho of sea level at the same
>> lat/lon.
>>
>> Finally, what is the rho of sea level near Ft Collins, CO? Obviously there
>> isn't one. What there is, though, is a value derived from a model of the
>> shape of oceans in the absence of continents, or ignoring continents. That
>> model is what is called the spheroid or the reference spheroid. As with the
>> representation of latitude and longitude, there are several used, and I
>> don't recall any of their names.
>> than the center of the earth? And this is the germ of my remark about
>> altitude from the center of the earth. And what would my altitude be? If I
>> were flying a plane, it would be my rho minus the value of rho *at the
>> ground* at my lat/lon.
>>
>> Now on to driving over a pass... The altitude I want to see is the altitude
>> above sea level, or rho at the car minus rho of sea level at the same
>> lat/lon.
>>
>> Finally, what is the rho of sea level near Ft Collins, CO? Obviously there
>> isn't one. What there is, though, is a value derived from a model of the
>> shape of oceans in the absence of continents, or ignoring continents. That
>> model is what is called the spheroid or the reference spheroid. As with the
>> representation of latitude and longitude, there are several used, and I
>> don't recall any of their names.
> [snip]
>
> The model is usually referred to as an ellipsoid as defined by the WGS80
> datum. The problem is that mean sea level does not follow the shape of the
> ellipsoid exactly but a much more irregular shape referred to as the geoid.
> See here for an illustration of the difference between the reference
> ellipsoid and the geoid:
> http://tinyurl.com/2fydf93
> The geoid being irregular can't be calculated but I believe that modern GPS
> units use look-up tables to find the difference between the calculated
> ellipsoid and the geoid at a particular position and hence calculate a value
> for the height above mean sea level.
>
> That's my understanding, but a true expert in these matters may come along to
> correct me...
>
> The model is usually referred to as an ellipsoid as defined by the WGS80
> datum. The problem is that mean sea level does not follow the shape of the
> ellipsoid exactly but a much more irregular shape referred to as the geoid.
> See here for an illustration of the difference between the reference
> ellipsoid and the geoid:
> http://tinyurl.com/2fydf93
> The geoid being irregular can't be calculated but I believe that modern GPS
> units use look-up tables to find the difference between the calculated
> ellipsoid and the geoid at a particular position and hence calculate a value
> for the height above mean sea level.
>
> That's my understanding, but a true expert in these matters may come along to
> correct me...
Thanks for the corrections to my terminology & my understanding.
--
Gene E. Bloch (Stumbling Bloch)
I recently travelled through Arizona, Utah, Colorado and New Mexico
and my 'cheap' Nuvi 250 was extremely close to the altitude signs I
saw, and that was anywhere from a couple of thousand feet to over
10,000 feet. I thought it wouldn't be accurate because of what I read
here, but it was with a few feet anywhere I checked it.
On Wed, 11 Aug 2010 14:22:36 -0700, "Gene E. Bloch"
>On Wed, 11 Aug 2010 14:41:01 -0500, Sunshine wrote:
>> On Wed, 11 Aug 2010 11:48:16 -0700, "Gene E. Bloch"
>>
>>
>>>On Wed, 11 Aug 2010 13:29:47 -0500, Sunshine wrote:
>>>>
>>>>>wrote:
>>>>>> Elevations are nice to know from the device itself. Maybe I should
>>>>>> have bought the 60CSX?
>>>>>GPSRs can get elevation data from the satellite fix, but it's relative
>>>>>to the imaginary sphereoid it thinks you're standing on.
>>>>
>>>> Why do you say it's relative? It's simply calculated, no different
>>>> from your Lat/Long, right?
>>>The spheroid defines the zero-point.
>>>The satellite data could give you the distance from the center of the
>>>earth, but that's not the usual reference for altitude; mean sea level or
>>>the assumed spheroid are possible references.
>>>>
>>>>>wrote:
>>>>>> Elevations are nice to know from the device itself. Maybe I should
>>>>>> have bought the 60CSX?
>>>>>GPSRs can get elevation data from the satellite fix, but it's relative
>>>>>to the imaginary sphereoid it thinks you're standing on.
>>>>
>>>> Why do you say it's relative? It's simply calculated, no different
>>>> from your Lat/Long, right?
>>>The spheroid defines the zero-point.
>>>The satellite data could give you the distance from the center of the
>>>earth, but that's not the usual reference for altitude; mean sea level or
>>>the assumed spheroid are possible references.
>>
>> Now I'm even more confused. Using signals received from multiple
>> satellites, the GPS receiver calculates a position in 3 dimensions.
>> Using map data, it should be able to calculate ASL or AGL, which I
>> think is what Bert was saying, but distance to the center of the
>> earth? :)
>>
>> Never mind, I'm going outside. ;-)
>> Now I'm even more confused. Using signals received from multiple
>> satellites, the GPS receiver calculates a position in 3 dimensions.
>> Using map data, it should be able to calculate ASL or AGL, which I
>> think is what Bert was saying, but distance to the center of the
>> earth? :)
>>
>> Never mind, I'm going outside. ;-)
>Please take your GPS with you so you don't get lost (no, that's not meant
>as sarcasm, it's just a wee bit of humor).
>When you come back in, read this;
>Even in two dimensions, the GPS only computes a position on the surface of
>the earth, which is (usually, at least) meaningless to us without a map to
>help us see where we are. And if we have different maps, we might see
>ourselves in different places, since all maps have inaccuracies. And in
>fact there are different representations (approximations) used, so that one
>map might translate the same position into, say, 72 degrees 31 min 42.92
>sec W, while another might give it as 72 degrees 31 min 42.61 sec W. In one
>case you may be on Main Street and in the other on Elm Street. Such a
>representation is called a "datum", and you can see which one your GPS
>uses, if you can find it in the menus.
>In the meantime, we are now faced with what an altitude is. If you're in an
>airplane in flight, you might be most concerned with your altitude above
>the ground (if not, I won't fly with you). When I'm driving a car over a
>pass, I'm pretty sure of my altitude above the ground, and I'm more
>interested in my altitude above sea level. But what is sea level in, say,
>Colorado?
>The GPS device calculates a position in some coordinate system. It hardly
>matters to us what is used internally, as long as it is converted
>externally into a representation that is useful to us.
>If I were designing such a system, I would use, internally, physicists'
>polar coordinates, where a position is given as a distance (called r or
>rho) from the origin, plus two angles, one *similar to* latitude and the
>other *similar to* longitude.
>And where would the origin of coordinates be? What could be more logical
>than the center of the earth? And this is the germ of my remark about
>altitude from the center of the earth. And what would my altitude be? If I
>were flying a plane, it would be my rho minus the value of rho *at the
>ground* at my lat/lon.
>Now on to driving over a pass... The altitude I want to see is the altitude
>above sea level, or rho at the car minus rho of sea level at the same
>lat/lon.
>Finally, what is the rho of sea level near Ft Collins, CO? Obviously there
>isn't one. What there is, though, is a value derived from a model of the
>shape of oceans in the absence of continents, or ignoring continents. That
>model is what is called the spheroid or the reference spheroid. As with the
>representation of latitude and longitude, there are several used, and I
>don't recall any of their names.
>Note too that this is the same thing the person who makes up the roadside
>sign that says "Rattlesnake Pass - Alt 1976 Ft" must do, and this was so
>way before GPS came to be.
>BTW, latitude and longitude aren't the only coordinates used. One that I
>once was familiar with is UTM (Universal Transverse Mercator), used by the
>military, among others. There are several of those too, with different
>origins, etc.
>If you still don't get it, I have nothing further to offer, since I limit
>myself to one major essay a week :-)
>In truth, it's really more a result of the fact that I can't think of how
>else to describe it.
>as sarcasm, it's just a wee bit of humor).
>When you come back in, read this;
>Even in two dimensions, the GPS only computes a position on the surface of
>the earth, which is (usually, at least) meaningless to us without a map to
>help us see where we are. And if we have different maps, we might see
>ourselves in different places, since all maps have inaccuracies. And in
>fact there are different representations (approximations) used, so that one
>map might translate the same position into, say, 72 degrees 31 min 42.92
>sec W, while another might give it as 72 degrees 31 min 42.61 sec W. In one
>case you may be on Main Street and in the other on Elm Street. Such a
>representation is called a "datum", and you can see which one your GPS
>uses, if you can find it in the menus.
>In the meantime, we are now faced with what an altitude is. If you're in an
>airplane in flight, you might be most concerned with your altitude above
>the ground (if not, I won't fly with you). When I'm driving a car over a
>pass, I'm pretty sure of my altitude above the ground, and I'm more
>interested in my altitude above sea level. But what is sea level in, say,
>Colorado?
>The GPS device calculates a position in some coordinate system. It hardly
>matters to us what is used internally, as long as it is converted
>externally into a representation that is useful to us.
>If I were designing such a system, I would use, internally, physicists'
>polar coordinates, where a position is given as a distance (called r or
>rho) from the origin, plus two angles, one *similar to* latitude and the
>other *similar to* longitude.
>And where would the origin of coordinates be? What could be more logical
>than the center of the earth? And this is the germ of my remark about
>altitude from the center of the earth. And what would my altitude be? If I
>were flying a plane, it would be my rho minus the value of rho *at the
>ground* at my lat/lon.
>Now on to driving over a pass... The altitude I want to see is the altitude
>above sea level, or rho at the car minus rho of sea level at the same
>lat/lon.
>Finally, what is the rho of sea level near Ft Collins, CO? Obviously there
>isn't one. What there is, though, is a value derived from a model of the
>shape of oceans in the absence of continents, or ignoring continents. That
>model is what is called the spheroid or the reference spheroid. As with the
>representation of latitude and longitude, there are several used, and I
>don't recall any of their names.
>Note too that this is the same thing the person who makes up the roadside
>sign that says "Rattlesnake Pass - Alt 1976 Ft" must do, and this was so
>way before GPS came to be.
>BTW, latitude and longitude aren't the only coordinates used. One that I
>once was familiar with is UTM (Universal Transverse Mercator), used by the
>military, among others. There are several of those too, with different
>origins, etc.
>If you still don't get it, I have nothing further to offer, since I limit
>myself to one major essay a week :-)
>In truth, it's really more a result of the fact that I can't think of how
>else to describe it.
Left-is-Gauche wrote:
> 1. Where are these elevations coming from?
From your GPS. Each track point in current GPSs include time, latitude,
longitude and elevation.
Time, latitude and longitude are reliable.
The 60CSX has a barometric altimeter. Your altitude changes in a short
period of time are accurate. But unless you calibrated the GPS'
altimeter at the start of the climb, your base altitude may be way off.
(aka: your track may be showing you climbing from 1000 to 1500 metres,
when in fact you started at 500 and went to 1000. Same climb difference,
different base altitude)
Units without barimetric altimeters (like the 60CX) use GPS altitude.
There is no need to calibrate this, so your base altitude is always
"right", but GPS altitude is much less precise and varies up and down
even if standing still. So there may be cases where yo are climbing, and
your GPS is saying you are going down. But overall, it follows your
climb profile.
the 60CX is more likely to give you correct altitude (+- a few dozen
metres). The 60CSX, to get correct altitude, basically requires you
train yourself to always calibrate it when you turn the machine on.
And GPS units with barometric altimeters are useless in aircraft since
aircraft pressure tells the GPS you are at 8000feet altitude even if the
aircraft may be at 35,000feet.
> For the first time, I uploaded Track and WayPoint data from two recent
> hikes using my GPSmap 60CX and was able to plot a vertical profile of
> the hikes complete with elevations and distances over the entire 10
> miles of track data.
> Two Questions:
> 1. Where are these elevations coming from? =A0MapSource? There's nothing
> on Mapsource to indicate topography and elevations. I don't believe my
> GPSmap 60CX can determine elevations.
> 2. How accurate are these elevations, assuming they're coming from
> MapSource?
> Elevations are nice to know from the device itself. Maybe I should have
> bought the 60CSX?
> Many thanks for any light shed on this.
> hikes using my GPSmap 60CX and was able to plot a vertical profile of
> the hikes complete with elevations and distances over the entire 10
> miles of track data.
> Two Questions:
> 1. Where are these elevations coming from? =A0MapSource? There's nothing
> on Mapsource to indicate topography and elevations. I don't believe my
> GPSmap 60CX can determine elevations.
> 2. How accurate are these elevations, assuming they're coming from
> MapSource?
> Elevations are nice to know from the device itself. Maybe I should have
> bought the 60CSX?
> Many thanks for any light shed on this.
For a mapping program, I use Mytopo's (formerly Maptech) Terrain
Navigator. It computes elevation data from the USGS topo maps. My
Legend HCX does not directly calculate elevation, but it can be
inferred from the topo lines of the map display.
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> [snip]