Recommendations to shoot / determine a grade on proposed road

[/pine]

That is not correct GPS does figure elevations. It is the weak part of consumer grade GPS. Survey grade GPS does excellent with elevations but costs big bucks.

Yes it is correct...do some research and you will find that a "GPS" device can give elevations but the data (elevation) is NOT coming from anything to do with the satellites or the positioning system...once a location is defined the device uses the location and interfaces with a "digital elevation model" (basically a database)...which is a third party and is not really part of a Global Positioning System...

What you are referring to that costs "big bucks" is just more exact software that interpolates to more precise calculations between exact positions and known elevations...

 

[s219]

I think there is a misconception of how a GPS device/app determines elevations...
All a GPS can do is determine geographical location...for elevations it must use third party data (nothing to do with GPS) i.e, data from topographical mapping that is correlated and interpolated...That is...once a location is determined it (device/app etc.) then relates that position to known elevations.

Not quite accurate. GPS calculates your 3D position in space, think (x,y,z) coordinates. From there, the data is reconciled to a simple model of the earth, a simplification known as an ellipsoid. By default, GPS uses the WGS-84 datum that covers the whole earth. When GPS gives position and altitude, it is the latitude and longitude of the base point, as well as altitude off the elipsoid. It doesn't know anything about terrain or topography, only the ellipsoid. You can certainly transfer the simple position over to another datum (different ellipsoid, some are localized to better approximate local regions) or work it relative to a terrain model, but you don't have to. GPS is able to give position and altitude relative to the WGS-84 datum all by itself, with no knowledge of the terrain.

 

[s219]

Yes it is correct...do some research and you will find that a "GPS" device can give elevations but the data (elevation) is NOT coming from anything to do with the satellites or the positioning system...once a location is defined the device uses the location and interfaces with a "digital elevation model" (basically a database)...which is a third party and is not really part of a Global Positioning System...

At this point you have a surveyor (dodge man) and GPS software developer (me) telling you that you're wrong. As I said before, GPS gives 3D position in space, and that involved three coordinates. It does all that with satellites. Anything involving an elevation model is a separate follow on mode of post-processing, but is not required.

 

[/pine]

Not quite accurate. GPS calculates your 3D position in space, think (x,y,z) coordinates. From there, the data is reconciled to a simple model of the earth, a simplification known as an ellipsoid. By default, GPS uses the WGS-84 datum that covers the whole earth. When GPS gives position and altitude, it is the latitude and longitude of the base point, as well as altitude off the elipsoid. It doesn't know anything about terrain or topography, only the ellipsoid. You can certainly transfer the simple position over to another datum (different ellipsoid, some are localized to better approximate local regions) or work it relative to a terrain model, but you don't have to. GPS is able to give position and altitude relative to the WGS-84 datum all by itself, with no knowledge of the terrain.

Only to a degree...the elevations derived from digital elevation models are much more accurate....this the reason that GPS altimeter watches etc. have large margins of error...often in the hundreds of feet...!

 

[dodge man]

There is something called a geoid model that is used to calculate the elevation of the ground based on the ellipsoid height. So in that sense Pine is correct
the gps is using data to calculate the ground elevation. It is not using topo maps or any other kind database to figure elevations. The argument is kind of pointless since there is all kind of number crunching going on to figure a 3D position based on various data and models, wgs84 being another part of a model.

To the op. If you had a distance of 540 feet a 10% grade would be an elevation change of 54 feet, so you should be ok if your numbers are good.

 

[/pine]

FWIW...

A Digital Elevation Model (DEM) is a digital representation of ground surface topography or terrain with different accuracies for different application fields. DEM have been applied to a wide range of civil engineering and military planning tasks. DEM is obtained using a number of techniques such as photogrammetry, digitizing, laser scanning, radar interferometry, classical survey and GPS techniques. This paper presents an assessment study of DEM using GPS (Stop&Go) and kinematic techniques comparing with classical survey. The results show that a DEM generated from (Stop&Go) GPS technique has the highest accuracy with a RMS error of 9.70 cm. The RMS error of DEM derived by kinematic GPS is 12.00 cm.

Assessing digital elevation model uncertainty using GPS survey data
— Penn State

 

[ovrszd]

You guys are arguing with yourselves. Did you read his last post? He knows very well what he's dealing with. Sounds like it's doable.

Thanks to the OP for the update. Looking forward to the final result.

 

[dodge man]

Yeah, not sure where you dug that up but it really has nothing to do with modern gps surveying. They are talking about the accuracy of a DEM and the accuracy of the results, such photogrammetry, stop and go gps, etc. The DEM is the results of surveying. The stuff they are talking about with gps is outdated, such as stop and go (I have heard it called PPK) and is not a technique any modern surveyor uses these days.

GPS surveying and results is a very complicated subject, one that I have only a vague understanding of. The theory is pretty simple but obtaining results is complicated. There is a lot that is going on with the software and computing that is pretty complex, does not matter if we are talking about a cell phone, handheld gps or survey grade equipment.

In my day to day use of gps there was never any data being used other than geoid model to calculate elevations. The elevations are calculated using the gps data and getting correctionS from a base or bases that are set on points of known elevation. The geoid model is not even needed, it just increases accuracy of the results.

Sorry to get off subject for the OP.

 

[quicksandfarmer]

At this point you have a surveyor (dodge man) and GPS software developer (me) telling you that you're wrong. As I said before, GPS gives 3D position in space, and that involved three coordinates. It does all that with satellites. Anything involving an elevation model is a separate follow on mode of post-processing, but is not required.

Actually, GPS give a 4D position -- 3D plus time.

 

[mapper]

I am a recently retired land surveyor and the weak part of cheap hand held gps is the elevations. If the area is wide open, no trees you might get decent results but I would not count on it.

They make cheap hand held levels, like a small telescope, about 10 long. Usually no magnification. I would put lath in every so often, say 25 feet measured in, and start at the bottom and work your way up the hill. You will need to know how high your eye is and make a mark on the lath ahead of you with a helper and measure how high the mark is.

For example, your eye is exactly 6 feet high, you make a mark on the lath 25 feet away 1 foot above the ground. That means the ground is 5 feet higher in 25 feet which equals a 20% grade. Keep in mind you can not measure higher than you stand, so if the ground in 25 feet is 7 feet higher you will have to set lath shorter than 25 feet. Keep in mind it shoots level, not up or down hill. This method is not great but is very cheap. They also make hand levels that will shoot up hill and show a grade on a scale on the side but are hard to find these days.

Some counties and states have free topo maps available. You probably need a way to print them out to scale them though. The quality of there software varies also on if you can just get the measurements on your computer or need to print it out.

Of course a surveying or engineering company could survey and design something but not cheap.

A hand held clinometer can be found on Amazon for about $40, Abney levels are a bit more. Used both in my forestry days to spot road grades.