Creating points from azimuth and distance: COGO in the field
Some boundaries are described by bearing and distance, not by coordinates.
Field takeaways
- Old sketches and written descriptions give angles and distances, not coordinates.
- Azimuth is measured clockwise from north — which north matters.
- A traverse chain accumulates error; the last point is the weakest.
- A computed point can never be better than the point you started from.
- Handing over the result as CSV makes the computation checkable.
What is left when there are no coordinates
An old sketch, an expropriation plan or a hand-written boundary description usually gives no coordinates. It starts from a known corner and walks: this bearing for this many metres, then that bearing for that many metres. A traverse does exactly that: it takes a starting point, an angle and a distance, and computes the coordinate of the next point. That point then becomes the origin of the one after it.
Which north?
Azimuth is measured clockwise from north — but from which north? Grid north, true north and magnetic north are not the same direction. A few degrees between them is a displacement measured in metres over two hundred. Most old sketches never say which one was used. This is the most expensive mistake in traverse work, and it has nothing to do with arithmetic.
Error accumulates along the chain
A traverse is a chain by nature: every computed point is the origin of the next. A small error in an angle is multiplied by the distance and handed on to the following leg unchanged. The last point of a twelve-leg chain carries the accumulated error of the eleven before it. Where you can, close the chain on a second known point and look at the misclosure; that number tells you how well the computation held.
It cannot be better than where you started
Computed coordinates inherit the accuracy of the point you started from. If you began from the phone's own GPS fix, entering the angles to the decimal will not lift the chain out of metre-level accuracy. The logic in standard geodetic computation is the same: the arithmetic can be flawless, but if the point it rests on is not, neither is the result.
Making the computation deliverable
If you need the computed points in the delivery system, convert the selected features in bulk, and type in a single coordinate to convert as a check. Export the result as CSV: a plain text file following RFC 4180 means someone else can open your computation and check it. A computation nobody can check has not really been delivered.
Try these steps on your own phone.
MapLab Survey handles RTK/GNSS point capture, drawing, area and volume calculations and coordinate conversion, then exports to DXF, KML/KMZ, GeoJSON, Shapefile, GeoPackage, NCZ or CSV. Capture, calculation and export work offline. Live NTRIP corrections and online basemaps need a connection.
Frequently asked questions
Is azimuth the same as bearing?
Azimuth runs clockwise from north through 0-360 degrees. A bearing is described within a quadrant, such as N45E. The two convert into each other, but confusing them puts the point in the wrong place.
What if the old sketch does not say which north was used?
Compute the azimuth between two known points and compare it with the sketch; the difference gives you the offset of the north that was used.
How many legs can a traverse safely have?
The count alone does not decide it. Where you can, close the chain on a known point and look at the misclosure.
Can I stake out directly to a computed point?
You can, but its accuracy is bounded by your starting point and the measurements you entered. For critical work, verify on the ground.
Technical references
The field guidance in this article is aligned with the technical and official references below.
Related: Coordinate conversion app · Stake-out · CSV coordinate list · MapLab Survey