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I have about 2 acres of land, wooded and brushy. There was a survey and the corners have rebar.

I am working on a site plan for house construction. I have two related problems:

  1. Finding a spot on the land, by reference. E.g., 10' from the E boundary, and 40' from the S boundary.

  2. Marking certain features down on a map. E.g., a particular tree that I want to keep - where is it in relation to the borders?

Between the terrain and the brush, I can't run a string between the corner rebar.

I picked up a cheap GPS (Bushnell ONIX 350), but I don't think I can get enough accuracy out of it for this purpose.

At this point, I'd be happy to get measurements within 1 foot of error.

I'm looking for cheap solutions. Any ideas?

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5 Answers 5

up vote 7 down vote accepted

The budget solution is to calculate relative positions from a corner stake. You will need a compass, a clinometer, a tape measure, a notebook, and an assistant. Starting at the known location, take readings for azimuth, inclination, and distance from the base point to your point of interest. If the point of interest is distant or around obstacles, you can make a chain of survey points. In the series, take measurements from point A->B, B->C, C->D, etc., until you reach your destination. You can use simple trigonometry to determine the relative positions.

To maximize accuracy, take readings as close to the survey stations as possible and record readings both from A->B and B->A. Good equipment helps too; for such tasks, I use a SUUNTO KB14 compass and SUUNTO PM5 clinometer. A Brunton Pocket Transit would work for your accuracy requirement, and can sometimes be found at military surplus stores for cheap. A laser measurer, plumb bob/protractor, and hand compass might work too (note: I've never tried that particular configuration. Making a loop from a known location to point of interest and back to the known point, and then assessing the closure error can provide a helpful metric of accuracy).

Relative position calculation functionality and error assessment are common in cave surveying software. See for instance, Fountainware's COMPASS Cave Surveying software (shareware). Such software often has GIS data export too.

For more pointers, you might check out web resources on orienteering, cave mapping, or land survey. Getting precise positioning requires practice, training, and skill. However, for 1' (0.3m) accuracy in distances likely within a 2 acre (~8000m^2) area, a careful novice could get satisfactory results.

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If you can't find (or can't afford) a clinometer, you can make one out of a plumb bob. Or just use the plumb bob. –  mwalker Aug 18 '10 at 18:49
    
Basic surveying in a nutshell. Good explanation. –  Nathan W Apr 13 '11 at 21:28
    
Another popular free tool, used by surveyors & engineers, is Copan: underhill.ca/land-surveyor-software –  martin f Jan 6 at 11:39

Setting up anchored poles at the corner rebars could help with either getting a straight line across, or with optical triangulation once you have a line of sight. The accuracy will depend highly on your optical equipment in the latter case. I guess you could rent a theodolite (or a professional land surveyor) for the task if that is in budget.

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I agree. The only sure way to do this sort of setting out is with a survey instrument. Work out the bearing and distance and direct an assistant into position using the instrument set up over the rebar. I say instrument, not theodolite, because you could potentially get away with using a simple level (which often have angles marked on them). You'd also need a laser distance measurer or similar (or you can turn the levelling staff sideways to measure distance I believe). But a theodolite would be fine, cheapish to rent, and with 1 foot accuracy it should be easy enough for a non-pro to do. –  Mark Ireland Aug 18 '10 at 22:59

This sounds like it might be more of a landscape architecture problem than a surveying problem.

Google Sketchup allows you to enter relative coordinates.

There's a Pro and a free version.

Use that in conjunction with a Laser distance tool.

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I'm using Sketchup already. Can you expound on how you would get the numbers to enter as relative coordinates in Sketchup? –  Jay Bazuzi Aug 21 '10 at 16:19

You might be able to do some sort of cheap differential GPS survey by using two GPS units. You'd place one on top of a known spot (one of your rebar stakes) and then use the other unit to survey in the points you need to locate. Then in a post processing step you can determine how far the fixed GPS thought it was from the known location at the time you surveyed a point and subtract that difference from the surveyed point.

There may be software out there that can do the maths for you but I don;t know of any off hand.

However I suspect that if you want any sort of real accuracy you would be better off hiring a professional surveyor, I know that when I did my surveying course as an undergrad we failed to close a loop around the building by 18m vertically and nearly 100m horizontally.

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There must have been some serious blunders made there @iant ;-) (A "blunder" really is a technical term surveyors use for "large human error".) –  martin f Jan 6 at 11:42

We rent good differential GPS survey equipment. Works for us. But I agree with iant; having a professional survey done might be best. Renting good equipment may be less expensive but there is a learning curve.

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What kinds of places might rent differential GPS? I'll call around to local tool rental places, but maybe there's somewhere else I should look? –  Jay Bazuzi Aug 23 '10 at 18:05

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