Help Determining Elevation!

[Willie]

I am trying to determine the exact elevation of a spring on a farm I am thinking of buying. It is important, because it is just above a proposed lake site. My GPS does not appear to be exact, but within 22 feet, which isn't good enough. It is an older unit, by the way. Question is, how do I determine the exact elevation of this spring? How do surveyors do it? Is there some instrument that I can use? I don't want to spend a ton. Techies help!!!!!!!!!!!!!!!!

 

[Restrain]

Get a Geo Map from the USGS that has the topographic lines and overlay the property you want to buy over the map. Then rent a builders level (the tripod type) and shoot however many lines it will take from an estimated known elevation point to your spring. That will give you the spring elevation. However, you can also just make an estimate from the topo map.

Or you can spend $300 and have a surveyor do it.

 

[Dave Smith]

Topo maps may get you closer than the GPS to actual elevation. My new GPS isn't exactly accurate for elevations either. Sometimes it tells me I am below lake level on Lake Michigan.

Teraserver and Mapquest should both give you topo maps. Your county should have topo maps. Local engineering firms usually have good topo maps. A surveyor could pin the elevation down to inches of actual.

Good luck.

 

[Carnivore]

How far is the spring from the pond?

Why do you need to know this?

From a former military pilot:

altitude/elevation can be the same, and are commonly measured from MSL(mean sea level) adjusted for local barometric conditions.

For example, the top of Mt Everest is 20,000+ ft above MSL. Your spring is X feet above MSL and your proposed pond has an infinite number of spots above MSL. New Orleans and parts of Death Valley are below MSL.

This is what surveyors use by means of bench marks. Bench marks have lat/long coordinates and MSL. Surveyors donot need to know barometric settingsbecause the bench marks when established were corrected for that at the time they were set.

 

[Thomas N. Morgan]

William,

I once talked to a surveyor friend of mine about using topo maps to establish elevations. He laughed. Said, not accurate.

The way surveyors do it is to go to a USGS "Benchmark". It is a brass plate set in concrete at ground level that tells the elevation at that particular point. From there, they shoot elevations to the subject site. I've never seen this last part done, but I do know the first step is to locate the nearest Benchmark.

That's the way it's done in Florida, and probably all over the US.

I know it should be obvious, but would you explain again in some detail why this is so important? What, exactly does that proposed lake have to do with the spring? Do you have any information about the designed maximum water levels for the proposed lake? If the lake level doesn't get too high, it could be a benefit: Might increase the flow of the spring, due to hydrostatic pressure.

I know that topo maps are good for getting the general lay of the land, and I think the preparers of FEMA Flood Maps use them, too. But, as far as I know, only a surveyor can give you anything like what you are looking for.

Regards,

Tom

 

[Mike Garrett RAA]

Topo map is the best in my part of the country. Use your GPS to locate the exact location on the map. I was working with an IFinder (lowrance GPS) this past weekend and it's altitude was within 10' of the map. Here is the problem with GPS...you can't adjust it for barometric pressure. All pilots know one of the first things you do in pre-flight is "set the altimeter" by adjusting the barometric pressure.

That little IFinder GPS is a remarkable piece of equipment. It is the same one the military issues to the troops (commerical version). We were able to get within 3 meters of exact spots on the lake. There are commercial grade GPS that can be adjusted for barometric pressue...surveyors probably use those. I think the Lowrance Air Map GPS also has that setting available.

 

[Willie]

Andrew, Thomas,

Why is the exact location important? Good question. The lake elevation should be below the spring site as it is the primary source of water. I am wanting to build as big a lake as possible. If the lake is over the spring site, in this area, there is a real possibility the water pressure will blow out the spring and forever send it elsewhere in the ground. Old timers around here say" don't mess with a spring", ie dig around or put undue pressure on it, as 5 acres of water might do.

 

[Pat Butler]

Most consumer quality GPS units aren't that accurate with elevation because many of the satellite signals will be aimed nearly at a tangent to the earth's surface. So the GPS unit might be trying to calculate your elevation by determining the difference between signals that may be only 1 degree apart. The GPS signals that are directly above you (say 90 degrees) are less useful for elevation determinations.

Topos aren't much more accurate unless you are lucky enough to have a benchmark nearby. The elevation lines are actually traced onto a map by hand as the cartographer views stereo aerial images. Sometimes they're pretty close- other times they can be way off.

How do surveyors do it? The use post processing procedures on their GPS data. Their GPS (aka field station) may take thousands of waypoints during a few minutes of data acquistion. That helps with averaging. Later, they will post process those files against information that provides 'corrections' against various atmospheric conditions and other errors. That's the best way to get an accurate elevation. If you do have a benchmark nearby then any old surveyor (even without GPS equipment) can determine an elevation directly from the benchmark.

Mike-- GPS units don't use barometric pressure. They determine elevation by triangulation. That's why they don't need to compensate to barometric pressure. From another old pilot...

 

[Bobby Bucks]

William there is no need to be so cryptic about the spring. There's nothing in USPAP that says you can't put a still on your own property.

 

[xm39hnu]

William,
If 20'± is going to make a difference, it sounds to me like the only accurate way to do it is to hire a surveyor. If you're building the lake, why quibble about a $300 survey fee to assure that the spring isn't damaged? Surely the spring is worth $300.