Surveying boulders on the surface

Speleotron

Speleotron

Member
There's a hill in the peak district covered with a few hundered boulders which are arranged in about ten groups over an area of a about a square mile. Fortunatly there is a trig point in the middle of one of the clumps that can act as a point of reference for everything.

How should I go about making the survey? The idea I have at the moment is to treat the trig point as the origin and, for each of the groups of boulders, take the distance and bearing of a prominent member of the group, from the trig point. Then for each group, take the distance and bearing of each boulder from the prominent boulder that I chose to take bearing of from the trig point. This will give me the position of every boulder in a group with respect to a 'chief' boulder, and the position of each chief boulder from the trig point.

But is there an easier way?

Cheers.

By the way its not for a cave survey its for a climbing guidebook to routes on the boulders.
 
SamT

SamT

Moderator
Hmm - sounds like Robin hoods to me, probably wrong though.

There has to be a standard procedure for mapping from a trip point. Try wikipedia.

easier way  :-\
Google earth??
 
Brains

Brains

Well-known member
1. Old fashioned way - triangulate between as many boulders and the trig point as you can see from any point. Accurately measure one leg, your base line. Apply maths and get the 3D survey...
2. Get the best GPS unit you can beg steal or borrow - and in your best Time Team mode get a fix on each boulder or part of boulder. Download and compute...

Either way, it will give a better survey than the exploding firework plot you descibe as everything will be interelated and errors should stick out like a breastwart
 
M

martinr

Active member
Plane table

Simple instructions here  or  here (or try google for a better link)

The great advatage of PT is that the only measurement you have to make is the length of the base line - no other linear measurements, and no measurements of angles at all.
 
P

Peter Burgess

New member
If its for a guide, then I wouldn't be too bothered about minor inaccuracies as long as the representation is reasonably faithful.
 
K

khakipuce

New member
If you have a high res image why not just use a mouse rather than a digitiser? But then it depends what you want to do with it - it would presumably be a derivative work of the original image and covered by their copyright.

Re plane tables - if you can get hold of a plane table alidade and staff you only need to set the table up at one position
 
I

IanWalker

Active member
Speleotron said:
There's a hill in the peak district covered with a few hundered boulders which are arranged in about ten groups over an area of a about a square mile. Fortunatly there is a trig point in the middle of one of the clumps that can act as a point of reference for everything.

How should I go about making the survey? The idea I have at the moment is to treat the trig point as the origin and, for each of the groups of boulders, take the distance and bearing of a prominent member of the group, from the trig point. Then for each group, take the distance and bearing of each boulder from the prominent boulder that I chose to take bearing of from the trig point. This will give me the position of every boulder in a group with respect to a 'chief' boulder, and the position of each chief boulder from the trig point.

But is there an easier way?

Cheers.

By the way its not for a cave survey its for a climbing guidebook to routes on the boulders.
Click to expand...
Sounds like your idea would be easiest.  Reasonably quick and accurate and you can check visually with a map or aerial photo once you're done.  Also having the bearings and distances on the topo might be of use to people looking for a particular boulder (eg from trig to chief 500m at 156deg and from chief to boulder 62m at 75deg).

Perhaps consider making a ring of measurements from chief to chief - this would allow an accurate check for boo-boos.  It's essentially a step towards Brains' triangulation.

Let us know how you get on!
 
I

IanWalker

Active member
A further thought; for where there are many boulders locally, you could set out a string line on a known bearing (could be north, 090deg or along an obvious path at 273deg).  Then measure the distance along the string and a perpendicular offset to each boulder (you can guess the right angle fairly accurately).  This should be accurate for boulders close together and is very, very quick.

And it's probably worth trying whichever method in your garden or round your street before you go on the hill.  That way you can make your mistakes there and you can work out what order you want your notebok columns in etc.  All simple stuff but helps you get going in the field - useful for short daylight hours!

Ian.
 
P

Peter Burgess

New member
Baseline / offset method is nice and quick, but my experience is that offsets of more than 5 metres result in increasingly bad errors as you misjudge the right angle. Also, beware of significant vertical displacements. For offsets more than 5 metres, just triangulate by running a measure from the point to two points on the tape, significantly separated along it to make a nice "chunky" triangle. Sorry for the technical terminology.
 
Speleotron

Speleotron

Member
Thanks everyone, at the moment it looks like I'l triangulate to get the location of the chief boulders then either do a bearing/distance plot within each group, or climb the highest one and draw the group if its small enough.
 
I

IanWalker

Active member
Speleotron said:
Thanks everyone, at the moment it looks like I'l triangulate to get the location of the chief boulders then either do a bearing/distance plot within each group, or climb the highest one and draw the group if its small enough.
Click to expand...
8)
 
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