Performing A Resection With Map and Compass

Blake

Nest In the Hills
Staff member
Administrator
Joined
May 2, 2011
Messages
3,655
Reaction score
247
Location
Central West, NSW
Here is a quick tutorial for anyone who would like to know how to do a resection with a map and lensatic compass.

------------------

Take a look at the location below. We are overlooking Mudgee in this case. Im using the town as its easier to understand known points. In the bush you want to use distinctive land formations like unusual geography, mountain peaks, hills, corners of water courses etc. In order to perform a resection you need to positively ID 2 but preferably 3 points on a map and with your eye. The image below is a panorama of our view..



Its hard to show all the detail in one image so I have marked a few landmarks they are, left to right:

1. A Dam
2. Twin Peaks
3. Runway & Stadium
4. Mt Buckaroo
5. Mt Frome
6. Telecom Tower

Lets pull out the binoculars and have a look at those close up.



These are our landmarks. For an accurate resection you want to try and get a good spread. In this case we are on the side of a mountain so only have 180 degrees to work with so I'm taking the dam on the left, the stadium in the centre and the telecom tower at the far right. For my measurements im using a lensatic compass because they are much more accurate for doing resections and recording bearings taken from real world features.

IMGP4963.jpg
IMGP4955.jpg
IMGP4958.jpg

Next we sight our landmarks with the compass. Ensuring the compass is level and moving freely. Our results are:

Dam: 352
Stadium: 40
Tower: 109

Its imperative that you are sure about your landmarks or this will not be accurate. Below are our three landmarks on the map. You can see in the first image there are a number of dams which can change size and shape depending on water. I confirmed this as our dam has some structures around it on its east side which show as black squares on the map. They were the only structures near a dam in this area.

Also Ive used the edge of the stadium when taking the reading which is more accurate that just shooting in its general direction so I will need to plot from that same edge later on.

IMGP5029.jpg
IMGP5036.jpg
IMGP5031.jpg

Next we lay out our map flat on the ground and orient it. I assume you know how to do this. Making sure you include the magnetic declination. In this case 13 degrees East. Our map is now oriented. I like to do it this way and use the compass like a protractor rather than have to calculate the back azmuth and plot it with a protractor. Less tools, less steps, less chance of error.

So we have an oriented map and our 3 azmuths for the Dam, Stadium and Tower.

Simply place the upper left corner of the compass on the respective landmark and rotate the compass (using that edge as the pivot) until your bearing is displayed over the index line. Now draw a line using the compass as a ruler.

IMGP5026.jpg

Repeat with the next two landmarks. All three lines should more or less intersect. They wont always intersect perfectly. Thats not uncommon. It should create a triangle and you are somewhere inside that triangle.

IMGP5037.jpg

Our location circled where the three lines intersect. I have written our coordinates below on some paper according to the map. They are 739400E 6389700N

IMGP5044.jpg

Lets see what the GPS says

IMGP4972.jpg

10m out in the east X 42m out in northing. Accounting for GPS inaccuracy you cant ask for better than that. Very unlikely you will get that in the bush. Anything under 100m is good. Your not directing an airstrike just trying to figure out what ridge or spur you are on so most of the time thats the only accuracy you need.

Cheers. Hope it helps someone.
 
Last edited:

barefoot dave

Mors Kochanski
Joined
Jan 1, 2014
Messages
392
Reaction score
22
Well done, Blake.
Easy to understand with effective pictures and great tips.
 

Mickldo

Ray Mears
Joined
Oct 28, 2012
Messages
424
Reaction score
1
Location
Maryborough, Qld
Great tutorial Blake.

I always used to get the terms resection and triangulation mixed up. Triangulation for those of you that don't know is a similar technique for working out the position of something you can see but can't get to (yet). That confusion existed until I heard the ditty "resection is ME-section", meaning resection is for finding out YOUR position, whereas triangulation is for finding the location of something else.

For a quick example of where you would use triangulation consider this scenario. You are hiking along a ridge top and you see an SOS from a signal mirror coming from the next ridge over. There is no way to get across the valley/river so you raise the alarm to the SES. To give them the co-ordinates you triangulate the position using a similar technique to the resection one Blake posted, but this time you take three or so bearings across to the signal mirror position from a series of known positions along the top of the ridge you are on. The greater the distance between you bearings the more accurate the triangulation will be.

Another scenario is you are on a beach and you see a ship sinking out at sea. You triangulate the position from a series of known positions along the beach so that once the ship finally goes down the search crews know where to start their search.

I hope this rambling of mine makes at least 1% of the sense that Blakes post does. Are you going to do Triangulation too Blake?
 
Last edited:

Blake

Nest In the Hills
Staff member
Administrator
Joined
May 2, 2011
Messages
3,655
Reaction score
247
Location
Central West, NSW
Great tutorial Blake.

I always used to get the terms resection and triangulation mixed up. Triangulation for those of you that don't know is a similar technique for working out the position of something you can see but can't get to (yet). That confusion existed until I heard the ditty "resection is ME-section", meaning resection is for finding out YOUR position, whereas triangulation is for finding the location of something else.

For a quick example of where you would use triangulation consider this scenario. You are hiking along a ridge top and you see an SOS from a signal mirror coming from the next ridge over. There is no way to get across the valley/river so you raise the alarm to the SES. To give them the co-ordinates you triangulate the position using a similar technique to the resection one Blake posted, but this time you take three or so bearings across to the signal mirror position from a series of known positions along the top of the ridge you are on. The greater the distance between you bearings the more accurate the triangulation will be.

Another scenario is you are on a beach and you see a ship sinking out at sea. You triangulate the position from a series of known positions along the beach so that once the ship finally goes down the search crews know where to start their search.

I hope this rambling of mine makes at least 1% of the sense that Blakes post does. Are you going to do Triangulation too Blake?

Thanks Mick. Good post.

Yep I can do one of those also if everyone wants.
 

Bloffy13

Jon Muir
Joined
Apr 11, 2012
Messages
2,805
Reaction score
120
Location
Albany, Western Australia
You can do a single leg resection if needed.
Let's assume you are walking along a known track, aiming for a set point say a track junction. You have been walking for a while and stopped to smell the flowers, enjoyed watching some eagles on high etc and you have lost track of time and just exactly how far you have come but now it's late in the afternoon and you need to work out whether you will make your destination before dark.
In the distance is a distinctive landmark. For convenience it is a phone tower on the top of a hill.
Take a bearing to the landmark, work out the back bearing and plot the angle from the tower along your map. Where it crosses the track you are on is your position. From there, you can work out the remaining distance and decide whether you need another night out bush or will make your car before dark.
Damn it, too far.
Guess it's gunna be another night out bush. I know I have phone reception because I have line of sight to the tower so a quick call home to tell the wife the battery is nearly dead but will be home tomorrow. Love Ya. Bye......click, brrrrrrrr.
Cheers
Bloffy
 
Top