contour plans surveying, suspicious or am I a cynic?

piranha · Feb 12, 2008 3:11 pm

I receive a contour plan for the block of land we purchased that was done by the developers 4 years ago before the roads were even made and have been informed that things might have changed over the year and the original contour might be inaccurate and I should get a new one done.

I had arranged this and received the report today, after comparing the new contour with the original 4 year old one which was done by a different survey company I notice nothing has changed. In fact it is exact to the mm each beoundary length is correct to .001 of a metre and all the contours are exact, I would have though even if erosion didnt occur that with new measurements there would be some difference of a few mm for the boundary how could they be so precise?

Now I know the new surveyors had access to the original contour plan as they have provided me with other surveying done of the street I get the feeling maybe they did a quick comparison notice there wasnt much if any change then decided to save them selve a few hours and just copy the original.

Am I being cynical here? or is surveying that precise? I know they use lasers which are pretty acurate but that would depend on exactly where they place their dumpy and to get it right to the exact mm seems a bit to precise.

stonecutter1309 · Feb 12, 2008 3:22 pm

Could it be that they use GPS to prepare the land to the original specifications they gave you?

We are in the same position - we have contour docs from 19 months ago - I wonder what it will look it in the end!

Talinsmum · Feb 12, 2008 6:23 pm

it could definately be an error - the block we have when looking at the contour map should have a 6m back to front drop on it over the building envelope and 8m over the whole block - effectively it would be building up a cliff! :shock:

In fact the actual block has 3m across the entire block of fall from front to back and the block is 58m deep) .... so yes errors do occur easily on the contour maps and ours were also done by the developers originally - I'd ask around for an updated version, or like in our case when the rep from the builder went out to assess the block he did his own contour map which was an actual reflection of the real drop of the block......

Fiona

piranha · Feb 12, 2008 6:35 pm

Quote:

I'd ask around for an updated version

We paid the surveyors to do update it, they surveyed the block last week thats my issue, its the same as the original contour so I question if they even did the measurements or just copied the original numbers and redrew everything.

piranha · Feb 12, 2008 6:40 pm

the contour says its a 7m fall front to back and is 30m deep, it just doesnt look that steep of a fall. the surveyours havent been the most professional as in the report the provided us 9 photos of someone elses block from the other side of brisbane, not one pic of our block and it doesnt state where the trees are in the diagram.

muzzman · Feb 14, 2008 8:02 am

showing photos of someone else's block??

yeah, i'd be cynical...

coljacqld · Feb 14, 2008 12:39 pm

does sound a little sus, our survey was done when the soil test was done so you may find your builder (if you have one yet) may have another one done.

One way to check the drop is to hire a dumfy level (if you want to fork out the money) that way if is not right you have a basis to challenge

Casa2 · Feb 14, 2008 2:50 pm

You can do it a lot easier than with a dumpy level. Just make up a water level. I did.

It consists of a tube (a garden house will do) that is long enough to stretch from the front of the block to the back (or wherever you want to measure). You can even use multiple hoses if need be. At the two ends of the hose, you need to put clear sections in. Now fill the hose with water ant tie one end to your reference point and take the other end to wherever you want to measure the height to. The water level at each end will be the same so if you measure down to the ground at each end then you will know the fall of your land.

piranha · Feb 14, 2008 7:51 pm

thanks for the tip casa but I just cant picture how that would work. a dumpy is expensive maybe just a long pole with measuemnts and a laser pointer would give me a fair idea.

Casa2 · Feb 14, 2008 9:06 pm

piranha

thanks for the tip casa but I just cant picture how that would work. a dumpy is expensive maybe just a long pole with measuemnts and a laser pointer would give me a fair idea.

piranha,

The water level works such that the water level at it's two ends are at the same level. Therefore by measuring from the water level to the ground at each end, then you know the height difference between the ends.

Another way, that you may like better, and is similar to the laser pointer idea, is just to use a normal spirit level and a very long pole. Put the pole on the low end of the block and stand with the spirit level on the high end of the block. Look along the spirit when it is level and note where your line of sight is on the pole. Again measure to the ground from pole's line-of-sight and the spirit levels distance to the ground.

By the way, both the water level and spirit level methods do not have to be done in one go. You can measure to one or more intermediate points and add up the results.

Hope this makes sense.

Cheers,
Casa

Yak_Chat · Feb 15, 2008 9:06 am

This might help.

Casa is talking about the same concept - but without the bucket in the middle.

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http://www.factsfacts.com/MyHomeRepair/WaterLevel.htm

Steve

Casa2 · Feb 15, 2008 11:55 am

The bucket is a good idea since it saves on having to top up the end to get the water back up into the clear end sections.

BTW, the way I did it in practice was a bucket at one end with a clear tube coming outof it. I could then take the clear tube anywhere I liked and I knew that the water level at the end was the same as the water level on the bucket. The survey plan I got and checked, turned out to be very accurate.

piranha · Feb 15, 2008 2:26 pm

oh ok, thanks for the pic & link. I might give it a go

Perth_Euro · Feb 16, 2008 12:54 am

As far as boundary lines go it is very feasable that they will match to the nearest mm.

The contours I would sugest should be different depending on where the measurements were taken internally and how they are interpreted.

Cadastral surveying is a very legal orientated feild. These guys go through alot of training (over 5 years here in WA) and the legal implications of the work they do is very important. For the senior surveyor to sign off on the titles etc, he put his "balls on the line"
(I studied surveyor for 2 years, didn't go down the cadastral path as its full of grumpy old men. Got into mine surveying instead.... "nearest 1/2 metre's good enough" lol)

It would be interesting to see if the boundary survey was done with GPS. I think probably not, as its not accurate enough in RTK mode. A post process would have needed to be done or traditional Total Station (jigger). For the internal heights to generate contours its possible GPS was used.