reading a compass

just surveying fenceline of my block. ( i already accidentaly bulldozed a heap of the neighbours bush already by trying to guess!!)

Using some trigonometry, the result is exceptionally small.

Depending on whether you measure 850 metres to the north, then move to the east until you make a reciprocal bearing of 180.00'20", or move 850 metres on your bearing of 0.00'20" prior to measuring your deviation from your north-south line from your origin, the amount of deviation can be represented as either 850.tan(0.00'20") which is approximately 8.2 cm, or 850.sin(0.00'00") which is also approximately 8.2 cm. Very difficult to measure over 850 metres!

I guess one also has to wonder how accurately with differing terrain one can measure 850 metres without specialised equipment.

Best of luck!

G'day qubert

Long time since I've done any trigonometry with sine & cosine but I agree with Charles. Deviation over 850 metres is 82 mm.

Regards

Presume you are not pulling the wool and are only after a very rough boundary?

I'm not a surveyor but I'd be surprised if your survey bearings/boundaries were based on magnetic north as the pole actually moves around from year to year ('polar wander'). More likely the readings are based on True North using a defined datum and coordinate system.

I agree 82 mm over 850 m, who would know?

Boundary surveys should be done professionally (far cheaper than legal fees sorting out the mess at a later date).

But just to show you the difference between Magnetic North and True North have a look at this diagram from Geoscience Australia :
http://www.ga.gov.au/image_cache/GA16046.jpg
Variations from -3 degrees in SW WA to +13 degrees in SE Tasmania.

Sorry, "seconds" of arc - 60 minutes in a degree, 60 seconds in a minute - it is 20/3600 of a degree or 0.00555555555555556 of a degree.

Pete (red faced)

I dont think you will ever find a compass with this sort of accuracy.
20" of arc is equivalent to bumping a theodolite or total Station.

Not to mention magnetic variations for a particular location

ob posted:
Not to mention magnetic variations for a particular location....

or the fact that the variations ( declinations) move over time at 1 - 10 minutes per year for any particlar location.

So even if you could find a compas that was 100 times more accurate than any other, and you had a map that shows the variation as of when it was printed, you would have to adjust for the changes over time at this level of accuracy. ( most good maps will show the movement over time).

If you picked say Canberra, it moves at about 5 minutes per year. So your fence measurement would move those 20 seconds in about 3 weeks.

So that is why the federal govt is in canberra. So it can keep up with the rest of us!