SparkFun Forums

[fll-freak]

@Lyndon: I stand corrected.

Plain GPS is indeed very poor in height even with excelent horizontal accuracy. And I knew that differential GPS was even better in the horizontal plane. What I did not realize was that they have been able to use DGPS to overcome the altitude problem. Now the page you linked to seems to indicate that you can only get those centimeter accuracies if your base station is close by (makes sense). If the OP is close enough to a base station for the accuracy needed, then that would be a solution. Of course, it does come with a price tag!

[UNTEngineer]

A quick google search yields this: http://en.m.wikipedia.org/wiki/Bathymetry

That talks about all the techs used to figure out your altitude under water, as well as sea rising.

Also take a look at http://www.oceanor.com/systems/seawatch ... nd-sensor/

[stevech]

I said differential GPS. DGPS is used to position the blades of earthmoving equipment with an accuracy of centimeters when used for final grading. Here is a table of accuracies of some off the shelf equipment. Sub-centimeter accuracies are possible.

I think you mean RTK GPS (real time kinematic; Google that). Surveyors use it. Sub-cm accuracy with long integration times. But not that good on elevation.

Differential GPS (DGPS) which is in-built in most modern GPS such as the Garmin OEM GPS 18 products (hocky-puck sized) ... Differential correction data is sent by geo-sync satellites and received by the DGPS receivers. DGPS corrects mainly for atmospheric errors (ionosphere). Coorrection data is also sent via the Internet and other radio means. Again, the elevation accuracy is not as good as x, y which can be less than 5m RMS at about 2 sigma.

[lyndon]

RTK GPS is a type of differential GPS. I haven't used it, but the table I linked to shows centimeter altitude accuracy for roving RTK and sub-centimeter for stationary RTK. However, I should have been more specific because there are other forms of DGPS that aren't as accurate. I think what you are describing on the GPS18 is WAAS, although I just learned that there is a type of DGPS that's not as accurate as RTK which uses satellite-transmitted correction signals.

I wish WAAS was as good as DGPS because I'm trying to find the lines on my property and I lost my copy of the surveying document that would let me do it by hand. With my WAAS receiver I can get close, but nowhere near as close as I need to be to find the property markers.

I said differential GPS. DGPS is used to position the blades of earthmoving equipment with an accuracy of centimeters when used for final grading. Here is a table of accuracies of some off the shelf equipment. Sub-centimeter accuracies are possible.

I think you mean RTK GPS (real time kinematic; Google that). Surveyors use it. Sub-cm accuracy with long integration times. But not that good on elevation.

Differential GPS (DGPS) which is in-built in most modern GPS such as the Garmin OEM GPS 18 products (hocky-puck sized) ... Differential correction data is sent by geo-sync satellites and received by the DGPS receivers. DGPS corrects mainly for atmospheric errors (ionosphere). Coorrection data is also sent via the Internet and other radio means. Again, the elevation accuracy is not as good as x, y which can be less than 5m RMS at about 2 sigma.

[stevech]

I said differential GPS. DGPS is used to position the blades of earthmoving equipment with an accuracy of centimeters when used for final grading. Here is a table of accuracies of some off the shelf equipment. Sub-centimeter accuracies are possible.

low cm and sub-cm accuracy with GPS requires RTK GPS, not DGPS.
DGPS is good to about 1-2m RMS.