AutoCAD Civil 3D General Discussion

AutoCAD Civil 3D General Discussion

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Distinguished Contributor
davevoith
Posts: 469
Registered: ‎08-09-2007
Message 1 of 11 (285 Views)

GPS datum question

285 Views, 10 Replies
07-21-2009 12:05 PM
Here is a question for all you surveyors out there. As I understand it the datum used for the Fema Maps is Navd 88. We need to make sure that out GPS is collecting data on the same datum.

NAD 83- Horizantal control datum(GPS default) IL-EAST-NAD83

WGS84- Ellipsoid (GPS default)

Illinois-03-Geoid model (GPS default)

NAVD88- Vertical Control Datum (used for fema maps)

I am stuck with the responsibility of trying to make sense of all this. I have read and read and read on it and can't seem to come up with a conclusion. Is NAVD based on NAD83? Is it the same datum? Please help a confused engineering tech.
Valued Contributor
CHRISHENDRICK5800
Posts: 77
Registered: ‎01-21-2008
Message 2 of 11 (285 Views)

Re: GPS datum question

07-21-2009 01:31 PM in reply to: davevoith
Dave,

Here in Texas, the FEMA maps are referenced to NGVD 1929 and the year of adjustment varies for different creeks, (some are based on 73 adj and other based of 79).

What we typically do, (and we have this factor for our area now), is locate a monument with a published value having the same datum and year of adjustment as FEMA. By doing this, we would now have a NAVD88, Geoid03 value for it as well and now know the difference. That difference can then be appied to you GPS collected points so they are based on the FEMA datum for your area.

To answer your last question regarding NAVD and NAD83 being the same thing....."NAVD" stands for North American Vertical Datum and refers to vertical. "NAD" stands for North American Datum and refers to horizontal. They are not the same thing.

There may be a way to set your GPS unit to collect in a particular datum and year of adjustment but we don't do it.
New Member
ttocshere
Posts: 1
Registered: ‎07-21-2009
Message 3 of 11 (285 Views)

Re: GPS datum question

07-22-2009 05:20 AM in reply to: davevoith
My question is what brand of Data Collector is being used and age of unit. With our TDS Ranger 500 I can use numerious different datums, from pre loaded to operator defined. An ez answer would be to talk to the company that sold the collector or the manufacture of the unit.
*Expert Elite*
AllenJessup
Posts: 5,928
Registered: ‎05-21-2003
Message 4 of 11 (285 Views)

Re: GPS datum question

07-22-2009 06:40 AM in reply to: davevoith
Start here: www.ngs.noaa.gov/faq.shtml
and here: www.ngs.noaa.gov/TOOLS/Nadcon/Nadcon.html
and here: http://digitalcommons.uconn.edu/cgi/viewcontent.cgi?article=1001&context=thmeyer_articles
and here: www.igic.org/standards/datum.html
GPS does not measure to the earth's surface. It measures to the Geoid. Then the Ortho Height for that location needs to be applied. I'm not sure about the capabilities of your equipment but we use Leica 300 & 500 units. The Ortho Height is applied durring post processing. We're usually doing Rapid Static observations. I believe you may be able to apply the corrections durring RTK but I'm not familiar with that process.
When I said that the GPS measures to the Geoid that isn't exactly true. GPS units actually measures the position in the system that the GPS satellites are navigating in, an earth based cartesian system that Surveyors would not be working in. My point is that anything you're getting out of your unit is based on a conversion. So it doesn't matter what elevation datum the GPS is collecting in. What matters is that you end up with your output in the correct datum.
Allen
*Expert Elite*
AllenJessup
Posts: 5,928
Registered: ‎05-21-2003
Message 5 of 11 (285 Views)

Re: GPS datum question

07-22-2009 06:54 AM in reply to: davevoith
This is a very nice PPT on the subject of Datums both Horizontal and Vertical.
www.ngs.noaa.gov/heightmod/3NMGPSHtsPrimerMay07.ppt
Allen
Distinguished Contributor
davevoith
Posts: 469
Registered: ‎08-09-2007
Message 6 of 11 (285 Views)

Re: GPS datum question

07-22-2009 08:31 AM in reply to: davevoith
Thankyou all for the help. We use the Leica System 1200 GPS. As I understand it the Geoid-03 converts the NAD83 ellipsoidal heights to NAVD88 orthometric heights. With this being said I think the unit converts the elevation data to be on NAVD88, but I'm not 100%. I'll have to read through those articles to get a better understanding.
Distinguished Contributor
davevoith
Posts: 469
Registered: ‎08-09-2007
Message 7 of 11 (285 Views)

Re: GPS datum question

07-22-2009 08:36 AM in reply to: davevoith
Right, I think the question is am I getting the correct output data. Those articles posted were more about converting form NAD27 to NAD83 and I'm not sure that is the case. Still confused on if the GEOID-03 corresponds to the NAVD88 datum.
*Expert Elite*
AllenJessup
Posts: 5,928
Registered: ‎05-21-2003
Message 8 of 11 (285 Views)

Re: GPS datum question

07-22-2009 08:37 AM in reply to: davevoith
The ultimate check when you believe that you have it set up correctly is to observe a know elevation. That way you can be confident that the results are what you expect. Also be aware that good mission planning can be critical when you need good elevations. You need to look for a constellation that puts some satellite at a high elevation rather than low to the horizon.
Allen
Member
jscheps
Posts: 3
Registered: ‎08-05-2009
Message 9 of 11 (285 Views)

Re: GPS datum question

08-05-2009 03:20 PM in reply to: davevoith
davevoith,

I am baffled that there are so many surveyors who seem to have such a lack of understanding about all the horizontal and verticle datums that are used in the GPS they turn on today, so it is no wonder that you would have a hard time with this. I may be saying some of the stuff Jesse already said, but I want to make this complete. Orthometric heights are heights that are referenced to a geiod, an equipotential surface closely identified with mean sea level. And this geoid is modeled by the verticle datum know as NAVD88. These height were done with leveling. Then came along good old GPS. Now GPS is actually referenced to the WGS84 reference frame. From the earths XYZ coordinate system (WGS84) we obtain LAT, LONG and an ellipsoidal height. Now this is confusing, but take my word here, anyone at NGS would (or should) tell you the same thing. All of our control is based on NAD83, and since NAD83 and WGS84 are so close, and since we GPS only figures out baselines in reference to other known control (when doing highly accurate work) all coordinates obtained are in NAD83. The only way to obtain true WGS94 coords is to autonomous positioning (+- 6-10m) or tie a project in to WGS84 control points obtaind from the Department of Defense (let me know when you get those too). Any height referenced to the ellipsoid is said to be an ellipsoidal height. Now, we still need to get to an orthometric height, so NGS came up with a model to closely represent MSL, and hopefully give you heights that are supposed to be Orthometric heights. So along come GEIOD03 (and others before that). Your software, I have no doubt, has this loaded so it is figuring out real time the difference between the ellipsoid and geiod. And the height from the Geiod to where you are on the ground would be considered an "Orthometric" height, supposedly heights representing NAVD88. I put orthometric in quotes for a variety of reasons, but for your purposes, you are using orthometric heights from GPS or NAVD88 heights, though I would not use these for any highly accurate work, unless you are running Static sessions to BM's close by, even then, leveling is the only true way to be sure if you ask me.
Distinguished Contributor
davevoith
Posts: 469
Registered: ‎08-09-2007
Message 10 of 11 (285 Views)

Re: GPS datum question

08-06-2009 05:59 AM in reply to: davevoith
Thankyou for your input, that is really good summary of all the mounds of data I have read to date.
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