1st April 2009, 01:36 PM
Nothing wrong with tape measures tied into a 1:1250 site plan. What is accuracy anyway, everything is based on tolerance.
I have an issue with these so called fantastically accurate GPS EDMs. How accurate are they anyway, I have had so many people expound on their millimetre accuracy which is based on defining your position in relation to a fixed point in space, x,y,z on the long lat grid.
The long lat grid was last updated in 1984 (WGS 84) and defines a set 3d spatial grid. Unfortunately our planet wasn?t consulted when they did this and not only changes shape as it spins though the solar system and wobbles unpredictably, its major land masses are not fixed in the relative positions to each other on the surface. 2cm continental drift a year in a roughly NE direction times 25 years, how accurate is the GPS system now? The point it is defining is slowly moving across the face of the planet. Same as 12 figure grid references based on the OS system of 1936. Is the Scilly Islands 0m E and 0m N still the same distance from us now as it was when OS was defined.
Doesn?t make all that much difference when tying in with tapes on a site map but is an issue when you are measuring things in relation to a fixed point in time and space when the item you are measuring isn?t fixed, doesn?t it?
Especially when you think that the archive we create is suppose to last for all of time. These fantastically accurate readings from modern GPS total stations are going to point to an area of open ocean in a million years time whereas the feature they would have recorded would have moved to the other side of Canada by then. What are we actually trying to achieve?
Rant over, for now
ten years on and still no bottom
I have an issue with these so called fantastically accurate GPS EDMs. How accurate are they anyway, I have had so many people expound on their millimetre accuracy which is based on defining your position in relation to a fixed point in space, x,y,z on the long lat grid.
The long lat grid was last updated in 1984 (WGS 84) and defines a set 3d spatial grid. Unfortunately our planet wasn?t consulted when they did this and not only changes shape as it spins though the solar system and wobbles unpredictably, its major land masses are not fixed in the relative positions to each other on the surface. 2cm continental drift a year in a roughly NE direction times 25 years, how accurate is the GPS system now? The point it is defining is slowly moving across the face of the planet. Same as 12 figure grid references based on the OS system of 1936. Is the Scilly Islands 0m E and 0m N still the same distance from us now as it was when OS was defined.
Doesn?t make all that much difference when tying in with tapes on a site map but is an issue when you are measuring things in relation to a fixed point in time and space when the item you are measuring isn?t fixed, doesn?t it?
Especially when you think that the archive we create is suppose to last for all of time. These fantastically accurate readings from modern GPS total stations are going to point to an area of open ocean in a million years time whereas the feature they would have recorded would have moved to the other side of Canada by then. What are we actually trying to achieve?
Rant over, for now
ten years on and still no bottom