Topo Survey 3

[ataman]

Hi,

I have a surveyor who is doing a topographic survey of about 200 acres of land. The land is not flat and has several undulations. The surveyor has generated a topographic map with 1m contour survey using a grid of spot heights of at 10-15 meter spacings.

My question is whether this spacing can give an accurate enough survey for doing road design, masterplanning etc.

If not what is the general recommendation.

I have attached a file for reference.

Thanks

 

[GoldDredger]

I think that should suffice for master planning and even concept road design. Although much more familiar with english units, it seems 1m contours at 15m grid spacing would work well enough.

For general master planning and concept layout, I would ask to have at least 5 foot contour elevations on a 100 foot grid of spot shots. That would be less precision than the 1m elevation with 15m spot shots you mention.

For final design, I would want to have 1' contours with a grid spacing not more than 50' apart. The surveyor should also pick up top and toe of slopes for undulations or unique features, to build an accurate surface (for your cut and fill quantities eventually). Also, the surveyor should pick up all utilites such as culvert size, location, elevation. Water and sanitary sewer appurtenances (valves, manholes, invert elevations of pipe). Power poles, utility pads, etc. Perhaps trees as well, if you have to mitigate their removal.

Don't forget the importance of an accurate property boundary boundary description as well. You wouldn't want to master plan a development only to find out later part of it is on some one else's property...back to the drawin' board!

 

[civilperson]

Using the same grid, ask for extra shots at toe of slope, top of slope and all prominent landmarks, (large trees, hydrants, power poles, paving, boundary corners and ties to exterior utilities such as sewer inverts and manhole rims). Have it plotted at 0.5 meter intervals for topo. This should be sufficient for all design and construction quantities.

 

[cvg]

this sounds fine for planning but woefully inadequate for final design. I would expect 1 foot contours at a plan scale of 1"=40' feet to be suitable for road and site grading design. If you are doing mass grading and since you say the ground is not flat, you should expect that your grading quantities estimates will be way off.

 

[ataman]

thanks for the responses.

Most persons have responded that closer contours would be recommended.

If I use the spot heights given by the surveyor, I can generate a topo to any precision.

I appreciate the comment about getting the heights at the top and bottom of slopes but with land that has this many undulations, it would be easy for the surveyor to say his has taken this into consideration and even more difficult for me to verify. Hence my question regarding a general grid size.

Thanks.

 

[cvg]

you can interpolate any contour interval you want with the existing survey data - but the accuracy will not improve. you need a closer grid spacing plus all the breaklines (flowlines, top and toe of all slopes) if you want to increase accuracy.

 

[francesca]

Typical grid spacing for 2' contours is 50'. With your 15 m grid you should be able to have 2' contours with suitable accuracy. I designed many a subdivision with a 2' topo, but we had an in-house survey crew who always staked our centerline early in the design process and allowed us to check the accuracy of our topo and adjust our grading if necessary. The topo never had us short on dirt, but getting the topsoil wrong did land us in trouble once or twice; fortunately all that meant was that for once the dirt contractor didn't try to skimp on the size of our retention basins!

 

[RWF7437]

Since no one else suggested this; plot the contours and go look at ( walk around)the site. Dots anything look missing or just plain wrong ?

 

[cvg]

I guess it comes down to how accurate you want your plans and your bids. You can do the design on the back of a napkin and get bids. but they will be high and you will have a lot of re-work to do during construction. Better survey and plans generally result in lower costs to construct and fewer surprises. Overall, just a better product. And perhaps cheaper to do additional survey up front than additional engineering at the back end.

 

[Hoagie]

Ignore the contour-precision stuff for the time being and use the topo for the schematic/preliminary design.

Once you get out of the SD phase of work then you will have a good idea where the road will be located. Have the surveyor revist the site and flag the centerline of the road--have him shoot station/offset topo from daylight-to-centerline-to-daylight. This should help you develop a tighter FG model and allow you to obtain cut/fill/net, confidently.

Good luck.

H.