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Datum Query: interaction between .gpx and mapping software

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#1
The Doomed Mapper

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Hello again all!

I had a question posed to me by one of my freelancers and I wanted to obtain a check on my logic.

When collecting data with a GPS, one has the ability to specify a datum in the unit. In theory, when the data is recorded and stored in the unit, it retains that data as part of the track file. I am curious how this datum interacts with the software environment when loaded into a mapping program.

For example, let's say I collect data in the field with a datum of WGS84 (the native coordinate system for .GPX data) on a recreational gps (an Garmin Oregon, for example). I then choose to import the data into mapping software of some kind (in this case, lets say Topo! State Series). This program also has the to specify a datum; in this case, Nad27, Nad83, WGS84. Normally, I would ensure that datums between the software and data matched, but what happens if there is a difference between the two? How is this difference resolved, and how will it affect the data loaded into the system? I'm more familiar with what would occur with projected data but "unproject" information such as this is a little less clear to me.

This feels like an elementary query, but my understanding is to never perform "on-the-fly" transformation on any data. I was wondering if the wisdom that held true for academic GIS hold true here. I am just curious if one datum "trumps" another.

~DM

I edited this post for clarity... please excuse the previous wording if unclear.

#2
jrat

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The data is still projected isn't it? WGS84 is only a datum. There still needs to be a coordinate system used (geographic or projected) that needs to be addressed. right? I would guess in your senarion that you need to make sure that the datum and coordinate system are the same between your gpsx data and your mapping software.

I have simmilar questions regarding Arcpad and the collection of gps data. With Arcpad I set up my feature class in ArcGIS desktop and check it out for editing. If my feature class has a projection of MD State Plane NAD83 ft and the GPS natively works in Parriee Carree WGS84. Is Arcpad doing the reprojection of the gps points on the fly?

#3
The Doomed Mapper

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The data is still projected isn't it? WGS84 is only a datum. There still needs to be a coordinate system used (geographic or projected) that needs to be addressed. right? I would guess in your senarion that you need to make sure that the datum and coordinate system are the same between your gpsx data and your mapping software.

I have simmilar questions regarding Arcpad and the collection of gps data. With Arcpad I set up my feature class in ArcGIS desktop and check it out for editing. If my feature class has a projection of MD State Plane NAD83 ft and the GPS natively works in Parriee Carree WGS84. Is Arcpad doing the reprojection of the gps points on the fly?


So perhaps this is all stemming from a fundamental misunderstanding of projected/unprojected data on my part: from what I understand, GPS coordinates are unprojected data although they possess a particular datum. Or is there an implied projection that I am not aware of (i.e. UTM?).

#4
jrat

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I think I was the one who is wrong. I misunderstood some of the articles I have been reading. I found this ESRI thread that helped my understanding of datums and projections. It might help you with your original question. Thread

#5
SpatialMinds

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This is a rough explanation, but hopefully it will suffice:

The earth is a big rock, not a perfect sphere or ellipsoid.

Spheres and Ellipsoids are used to approximate the shape of the earth

Latitude and Longitude are used to reference a point on a sphere or ellipsoid. Lat/lon coordinates are 100% meaningless unless the datum they reference is included along with them. It's like high school science where your teacher kept harping on you to include the units next to the numbers in your equations.

Traditionally, datums are a sphere/ellipsoid that is positioned in such as way as to minimize distortion in a particular place on the earth. That is why there are so many datums named after a local region such as North American Datum of 1927. Think of a perfect sphere being position in such a way to best match the outline of a rock in a specific spot on the outside of that rock. Everything is about getting the best fit in a particular area of that rock without any regard for the rest of it.

When the GPS system was launched they needed a datum that could be used globally, so they centered it on the gravitational center of the earth rather than repositioning it to get the best local fit. That's WGS84.

A map projection is the 3D data projected onto a flat surface. Originally this was done by literally putting a light bulb next to or inside of a globe (a scale of the earth) and projecting it onto a wall where it could be traced. As the science of cartography progressed they came up with mathematical projections, but it is fundamentally the same as the original light bulb.

Unprojected coordinates are often times referred to as being in the "Geographic" projection. This is very confusing, because they aren't projected at all. This confusion stems from the earliest days of ESRI software and was later adopted by the OSGEO Proj4 library as well with strong objections from the author of the original PROJ4 program. Both ESRI and the OSGEO version of Proj4 combine datum transformations and coordinate projection into one user level step. They are in fact separate operations and it helps tremendously to keep them separate in your head when working with coordinate data.

The GPX standard specifies that all lat/lon coordinates be relative to WGS84. Many GPS manufacturers get this wrong though and record the coordinates using the datum that the user specified for display on the GPS screen. You have to be careful to ensure you really know what you are getting.

Assuming the GPX data is in WGS84 as it should be and that you have a map in say UTM using the NAD27 datum (like old USGS quads use) then you need to do two things:

1. Transform the datum of the GPS data from WGS84 to NAD27
2. Project onto a plane: Project the 3D Geographic (NAD 27 ) data to 2D UTM

Hope that helps,
Craig

Edited by SpatialMinds, 10 November 2010 - 07:23 PM.


#6
Darren Rattai

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This is a rough explanation, but hopefully it will suffice:

The earth is a big rock, not a perfect sphere or ellipsoid.

Spheres and Ellipsoids are used to approximate the shape of the earth

Latitude and Longitude are used to reference a point on a sphere or ellipsoid. Lat/lon coordinates are 100% meaningless unless the datum they reference is included along with them. It's like high school science where your teacher kept harping on you to include the units next to the numbers in your equations.

Traditionally, datums are a sphere/ellipsoid that is positioned in such as way as to minimize distortion in a particular place on the earth. That is why there are so many datums named after a local region such as North American Datum of 1927. Think of a perfect sphere being position in such a way to best match the outline of a rock in a specific spot on the outside of that rock. Everything is about getting the best fit in a particular area of that rock without any regard for the rest of it.

When the GPS system was launched they needed a datum that could be used globally, so they centered it on the gravitational center of the earth rather than repositioning it to get the best local fit. That's WGS84.

A map projection is the 3D data projected onto a flat surface. Originally this was done by literally putting a light bulb next to or inside of a globe (a scale of the earth) and projecting it onto a wall where it could be traced. As the science of cartography progressed they came up with mathematical projections, but it is fundamentally the same as the original light bulb.

Unprojected coordinates are often times referred to as being in the "Geographic" projection. This is very confusing, because they aren't projected at all. This confusion stems from the earliest days of ESRI software and was later adopted by the OSGEO Proj4 library as well with strong objections from the author of the original PROJ4 program. Both ESRI and the OSGEO version of Proj4 combine datum transformations and coordinate projection into one user level step. They are in fact separate operations and it helps tremendously to keep them separate in your head when working with coordinate data.

The GPX standard specifies that all lat/lon coordinates be relative to WGS84. Many GPS manufacturers get this wrong though and record the coordinates using the datum that the user specified for display on the GPS screen. You have to be careful to ensure you really know what you are getting.

Assuming the GPX data is in WGS84 as it should be and that you have a map in say UTM using the NAD27 datum (like old USGS quads use) then you need to do two things:

1. Transform the datum of the GPS data from WGS84 to NAD27
2. Project onto a plane: Project the 3D Geographic (NAD 27 ) data to 2D UTM

Hope that helps,
Craig



In order to correctly transfrom GPS data from WGS84 to NAD27 or NAD83 with the least amount of error, please see the document below for a reference, it is ESRI specific though.

Transformations

Cheers,
Darren

#7
imran aziz tunio

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The data is still projected isn't it? WGS84 is only a datum. There still needs to be a coordinate system used (geographic or projected) that needs to be addressed. right? I would guess in your senarion that you need to make sure that the datum and coordinate system are the same between your gpsx data and your mapping software.

I have simmilar questions regarding Arcpad and the collection of gps data. With Arcpad I set up my feature class in ArcGIS desktop and check it out for editing. If my feature class has a projection of MD State Plane NAD83 ft and the GPS natively works in Parriee Carree WGS84. Is Arcpad doing the reprojection of the gps points on the fly?


So perhaps this is all stemming from a fundamental misunderstanding of projected/unprojected data on my part: from what I understand, GPS coordinates are unprojected data although they possess a particular datum. Or is there an implied projection that I am not aware of (i.e. UTM?).



WGS84 is best fit for all Attached File  GLOBAL2010_08_19_231.gif   75.7KB   33 downloads




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