Does anyone have a source for data on the ground temperature for a give location, lattitude, or other basis? I have an in-ground swimming pool. Can I measure the water temperature in the deep end and use that as a good indicator? I do not have a heater on the pool. Thanks in advance Tommy

http://smu.edu/geothermal/heatflow/surtemp.htm

The ground temp, below the frost line, for any area will always be within two degrees of the yearly average temp for that area.

Bergy

Two many variables on the deep end of the pool.
J

If you are running no circulation the temp at the deep end will be close to the ground temp assuming it is about 8 feet deep. But if you not running any circulation you likely have a swamp or pond and not a pool ;) The circulation pumps move the heat around pretty well.

Do you know what your average annual mean temperature (MAGT) is? (For Baltimore, MD it's about 57 deg F) Google on this if you don't know have it.

At a depth of 1/2 meter, the temperature varies 14 deg F. So, if your MAGT is 60 deg F, the ground will vary between 46 and 74 Deg F at a depth of 20 inches.
At 1 meter, temp varies 12 Deg F
At 2 meter, temp varies 7 Deg F
At 3 meter, temp varies 4 Deg F

You have to go down to almost 13 feet to get a temperature variation of two degrees fahrenheit

Obviously, this is just a guide.  Your mileage may vary.

Don't use your pool temp. Even without the pump, convection currents stir the water.

The best answer is "it depends"

What do you mean by ground temperature? 1 inch down, 10 ft down, 1 mile down?

Bergy's statement on the yearly average temp is generally true in most areas but as Hector pointed out the depth of the ground affects the temp swings.

Hectors info on variation is probably correct in his area but is based on the temp swings he gets. The 2 meter ground temp swings in south Florida might be a lot less.

And if you are in an area with geothermal activity near the surface then the seasonal temp change may be irrelevant.

http://www.geochoices.com/soil_and_ground_conditions.html

I wonder how far you have to go down before you start getting into heat from the liquid core of the earth?  With hot enough water you could by pass the geo system entirely. Would the electricity the pump uses be worth the effort? Can Tuffluckdriller or Douglas Walker give us drilling qoute per mile?  And if you drill too far, instant volcano?  This is kind of crap I wonder about.

Sure TG, and how many cups of sugar does it take to get to the moon?;)

Contact the state agency that issues permits for drilling water wells. They should have available well driller reports for all water wells drilled in the state, and maybe even have them online. The reports quite likely will have the water temperature when the well was drilled. Look for reports for wells on the order of 50 to 100 feet deep close to you. That's probably as good information as you'll find anywhere. Reports from various times of the year will give you an idea how much the temperature varies seasonally. Won't be much, if any. And most likely will be in the range of 50-60°F.