Transcript
Method for Determining True North Using a Single Handheld GPS
Purpose: This method serves as an alternative North finding method when the sun is not available, or as an emergency field expedient method. Site Requirement: From your base point, you must have an unobstructed view to your remote point either North/South or East/West for a minimum of 300 meters. Baselines longer than 300 meters will give more accurate results. If you cannot get a baseline of at least 300 meters, the accuracy will suffer. See the table at the end of this document. Tools and Equipment: a. b. c. d. e. f. g.
Theodolite Tripod Brunton® Compass Handheld GPS (no specific model) Carpenter’s Square Straight Edge Chalk Line
Procedure: 1. Make sure the GPS datum is set to WGS-84 (this is our standard ellipsoid model) and that the GPS is set to True North reference (not magnetic). 2. If the GPS supports ground based and satellite based differential error correction such as WAAS, EGNOS, or MSAS and the site is in a geographically supported area illustrated in the graphic above, enable it. This will improve accuracy even more. The accuracy can be down to < 3 meters 99% of the time. Without differential error correction you can expect accuracy of < 10 meters 95% of the time. 3. Select the area on your site where you would like to mark the North line. This is the “base” point. 4. Turn on the GPS and wait until it gets as many satellites in view as possible. Some GPS receivers report accuracy figures, a GPS constellation view, or a number related to the PDOP (Position Dilution of Precision). Accuracy less than 3 meters or a PDOP of 2 or less would be ideal. A wide spread constellation is good. A close together constellation is bad. 5. Once good accurate coordinates have been established, save the “base” coordinates as a waypoint and write down the coordinates on paper. We will refer to the waypoint and written coordinates later. 6. Set up and level the theodolite. Use the optical plumb to mark the base point.
7. Many handheld GPS units have an internal compass function and compass display. Pace off from the base point with the GPS in hand heading North (if unobstructed) for 300 meters or more using the compass display. You do not need to pace directly to your remote reference point. However, you must be able to see the base point from the remote point. Alternatively, you can pace off with a magnetic compass. 8. Using the navigation function of the GPS, call up the base waypoint and use the “goto” function to “find your way back” to the base point. The navigation function will show the distance and direction of travel from where you are standing back to the base point. 9. With the distance now acceptable between the base and remote point, in order to place the second mark (remote point), look at the coordinate display (not the compass display) of the handheld GPS. What we want to do is match the longitude numbers from the base point with that of the remote point. To do this, pace back and forth in an East/West direction. When the longitude matches the base point, mark the location so that it can be sighted with the theodolite telescope. A visible stake or rock cairn will do nicely. 10. Back at the base point; use the theodolite to sight the remote point. This is the True North line. Mark your North line by joining the base point with a near point marked with the aid of the telescope. 11. Let’s say that North was obstructed and you had to pace off to the South. We still have to match the longitude coordinates, but the difference is that we went South. We will either have to “flip” the theodolite telescope or turn the Theodolite 180 degrees to point to True North. 12. This time, let’s say that both North and South are obstructed but the East or West have open views. This time, we are establishing an East/West line. The difference here is that we must match the latitude coordinates of the base point with that of the remote point pacing back and forth North/South. Once establishing the East/West line it is only a matter of turning the theodolite 90 degrees to establish a true North line. No Theodolite Scenario Procedure: 1. What if you are faced with a scenario that excludes the use of a theodolite? How do you sight down range from your base point to the remote point? 2. This is an emergency field expedient method and should be considered when no other option exists. 3. Follow the above procedure substituting a straight edge, length of straight lumber, or pipe in place of the theodolite. This is your aiming device. 4. Establish a stable elevated platform of boxes/crates to place the aiming device on to. It must be at a height that allows you to look down the length of the aiming device to the remote point. 5. Attach improvised plumb bobs (string and nuts or bolts) at either end of the aiming device. The plumb bobs are used to translate the North line points from the aiming device.
6. Sight down the aiming device from the base point to the remote point. Mark the points from the plumb bobs and join the points with a straight edge. This is the North line.
Table 1. Probable errors in azimuth when determined by using the hand-held GPS method over a baseline. The maximum errors are up to 6 times as large as the probable azimuth errors shown in the table, but errors larger than the probable errors listed should happen less than 5% of the time. Though baseline lengths of less than 300 meters are shown, baselines of 300m or greater should be used.
GPS Accuracy Baseline Length (meters) (meters) 3 30 3 50 3 100 3 200 3 300 3 500 3 1000 10 10 10 10 10 10 10
30 50 100 200 300 500 1000
Probable Azimuth Error (95% confidence) (degrees) 1.9 1.1 0.6 0.3 0.2 0.1 0.1 5.6 3.6 1.9 1.0 0.6 0.4 0.2