How to survey brick chamber under house

If you only have the one access and no other way to see out...two plumb bobs hung opposite each other then line up on them down below. Old mining Surveying method. ?ÿThat gives alignment and one of them position.

Test the air before descending.?ÿ Confined space work can be dangerous.

Traverse into house and find two inconspicuous locations above chamber where you can drill through floor. Transfer locations to floor of chamber to establish a baseline. Locate the drill holes by traverse. Use the transferred locations as a baseline for taped measurements and sketch and measure from established baseline.?ÿ?ÿ

Handheld scanner. Should easily register to 0.1 meter or so

I've done my time in some pretty nasty areas, as I'm sure most of you have. This is suspect of a septic tank (I will pass ten ways until Tuesday), if that's the case there are plenty of utility locaters with GPR that can delineate it from the surface which may not be 50mm accuracy but from what I'm interpreting you are describing, is good enough.?ÿ

Make some preliminary measurements with hand tools to establish a search position, stake the search position in the garden, pothole the outside extense with a vac (contract that part out) locate what is uncovered.

Using a hidden point rod is especially handy for locating potholed features as the holes are never plumb.

Probably overkill for you application but this method should avoid any surprises.

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Depending on the thickness of the floor, and assuming your total station is reflectorless, you could set control on the bathroom ceiling, setup in the chamber directly below the manhole opening, relocate the control and trilaterate/triangulate your current location in the chamber.

NEVER GO INTO ANY CONFINED SPACE WITHOUT PROPER GEAR AND TRAINING!

(was that to loud?)

Is it too obvious to point out that if you have a accurate measure of the volume of water and depth of tank that you then have big clues to the other dimensional constraints.?ÿ

You don't have to even empty it.?ÿ Take a long pole stick it in at an angle move until you find the bottom corner.?ÿ Mark the ground elevation, and shoot the pole in 2 places where it is sticking out of the ground.?ÿ Use those 3 points to compute the bottom corner location.?ÿ Repeat as necessary, and plot in CAD in 3D.?ÿ If the pol will not go straight in, make a Z shaped tool and use the same method.

Rent a handheld scanner like geoslam or paracosm. Not as expensive as you think, but I wouldn't buy this job. ?????ÿ

Start by doing the math, 2000 liters reduces to 70.6 cu.ft. Comparatively if it were a 5' deep by 5' diameter tank it would be 98 cu.ft. Since you have exterior dimensions of said building measure with a tape to 4 points on the manhole rim normal to the basement walls. Mark those points with a lumber crayon or permanent marker. Using a 6' fibre (not wood) folding rule, measure pit depth then fold and measure to pit wall normal from marks toward cellar walls. You have enough information to figure out if the tank is round. If it is not round measure to the bottom interior pit corners and recalculate. Two straight line measurements from each manhole rim mark to each corner plus depth are all that is necessary. It would be very unusual for the tank to extend beyond the footprint of the dwelling. Precision required is 0.1'. Bricks laid in a circle are within that precision. Wash your rule thoroughly. What is more critical than size or location of this tank is the basement floor, pit bottom elevation and the invert of any lines that drain from this tank. For that the tank should be drained. No need to enter a tank of this size.

For a much larger tank use an extendable level rod rather than a folding rule.

Paul in PA