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Discussion Starter #1
I am marking some underground utilities via gps. Trying to figure out what accuracy/repeatability I'm getting with silly Tomtom while I wait for the goods to arrive...
if I mark an item at
N38* 43.692 and W120* 48.426
and return later to the same exact spot and get
N38* 43.696 and W120*48.426
How far off are my coordinates... How many feet etc does the last number equate to? Alot are the same reading but the worst are .004 off.
Help?
 

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I think that 1 degree is equivalent to 1 nautical mile. So the difference would be .004 knots, or about 24 feet. Of course, I could be completely wrong.

Edit - those are minutes and seconds, aren't they? Then shit, I don't know
 

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That is the number my GIS came up with for the distance between those points.

I think the .001°=24' is only good for latitude.
Distance between longatude points is going to vary as you move north/south along the line. 1° at the equator is going to be much further apart than 1° near the north pole.
 

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I don't think the GPS is accurate enough to really pinpoint a location like a utility. You need to be within a foot or two for that to really be effective. I have yet to see a GPS with accuracy more than about 10 feet.

Mine tells me, once it locks satellites, I get the accuracy in the corner. More satellites - higher accuracy. Best I have seen was 10 feet though. If you're lost that's more than good enough. If you're digging for your "gas line" it doesn't cut it.

Call USA and have them mark it for you!!!

If it's for your other "gas line", come up with something else. :flipoff2:
 

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I think the .001°=24' is only good for latitude.
Distance between longatude points is going to vary as you move north/south along the line. 1° at the equator is going to be much further apart than 1° near the north pole.

Correct.
 

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I don't think the GPS is accurate enough to really pinpoint a location like a utility.
Agreed. 2-10 meter accuracy from a typical civlian sporting unit is probably not going to be good enough for this.

How ever depending on the equipment used, it could be. Look at the degree of accuracy they are getting out of the unit used on road graders.

i read somewhere that Gen III w/spot beam they are talking accuarcy in mm
 

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Discussion Starter #10
Thanks for the input.
The unit I ordered is accurate to the centimeter (post download internal number crunching) assuming good satellite coverage. Accurate to less than 2.8' live while mobile.
Professional gps not camping junk.
 

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Thanks for the input.
The unit I ordered is accurate to the centimeter (post download internal number crunching) assuming good satellite coverage. Accurate to less than 2.8' live while mobile.
Professional gps not camping junk.

i was going to make a joke about your location being behind crystal view gas station but now it looks like you are at high angle driveline:confused:.
 

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I have bid out a fair share of underground utility mapping.

For water, I am looking +-1 foot accuracy.
You get me within a foot of a valve, meter, etc. I am going to find it with a locator.
You get me within 20-30' of a hydrant I am going to find it.
I want +-10' in elevation. That way I can look at the gradient of the system, look at the elevation of the meter and calculate the static pressure within +-3 psi.
Gages are only good to +-5 psi, so there is no need to worry about anything under a foot.

For sewer, elevations are everything, horizontal is about the same as water.
Horizontal is about the same as water. Put me within 3' of the center of the manhole lid, I'll be within a foot from the edge. If it is buried, I will be able to hit it with a hoe bucket.
In gravity sewers, elevation is everything. I require an accuracy based on grade of pipe. If it is going down a steep canyon, slope isn't a problem so I only need +-1 foot. If it is moderate slope, give me down to +-0.1 foot. If it is relatively flat, I need tight elevations +-0.01'. I can't tolerate more than a 0.01 foot when laying minimum grade sewers. GPS is only going to give you +-0.04' in elevation. So minimum grades must be surveyed using differential levels.

Here is the spec for the last sewer mapping RFQ I sent out.
GPS survey equipment shall be used to locate the physical facilities. The coordinate system used will be West Virginia State Plane, NAD83 Horizontal Datum, NAVD88 Vertical Datum, in U.S. Feet or U.S. Survey Feet.
GPS coordinate values must be scaled up to ground values. Both a copy (hard and digital) of the Grid Values and a Copy of the Ground Scaled values shall be included for records. However, the ground scaled values will be used.
The base position that the ground scaling was applied from shall also be noted for the record and for future use of mapping on that project. The File format shall be in the ASCII Text format, comma delimited, and shall be in this order: (Point #, North, East, Elevation, Description)

The accuracy standards shall be

• Horizontal shall be plus or minus one foot.
• Vertical shall be based on the pipe grades of both incoming and outgoing pipes as follows.
• Structures with gravity piping either flowing to, or from, the structures with a grade of less than 2% shall be plus or minus 0.04’ vertical accuracy.
• Structures with gravity piping either to, or from, the structures with a grade greater that 2% and less than 10% grade shall be plus or minus 0.1’ vertical accuracy.
• Structures with gravity piping either to, or from, the structures on obvious steep slopes shall be plus or minus 1.0’ vertical accuracy.

Physical facilities not possible, or feasible, to locate with GPS survey equipment must be located to the same accuracy standards as stated above.
 
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