What's the difference between bench marks and control points? 3

[fya]

What's the difference between bench marks and control points?

 

[BRIS]

Bench marks provide a level only (vertical control) . Control points have coordinates (vertical and horizontal control) .

 

[fya]

Thanks. but why would you need bench marks, why not only use control points if they give you both?

 

[sam74]

Most any contractor is going to have a level and level rod on site to check finish floors of buildings or the like by leveling off of the bench mark. So he can do this himself instead of having surveyors come out and charge him $500 for half a days work. A benchmark is typically for the builders. Control points are typically for those doing detailed layout and staking for the builders.

 

[StephenA]

In survey, bench marks need to be closer to each other than control points as horizontal angles can be measured more accurately than levels over longer distances.

 

[BRIS]

n addition to Sam74’s reply;

Maps will give you your horizontal position but not your height (you can interpret between contours but not accurately).
In UK bench Marks are located by the Ordnance Survey on solid structures etc at frequencies of about 1/2 mile and are shown on maps. When doing preliminary work such as locating pipelines it is easy to establish your horizontal position on a map but impossible to establish your vertical height hence the need for bench marks. You can simply make a reconnaissance survey of a utility route by following a line on a map and running a string of levels. By tying the levels into bench marks along the line you get a check on accuracy. I guess mapping in the US also includes bench marks. Often in UK setting out of utilities is made from existing features without the need for accurate horizontal control.

As Sam says when you first establish a site you bring in the surveyors to set up 2 or 3 control points for horizontal control. Once you have set out the plan it is fixed for the job but you need vertical control for everything from setting drain falls to roof slopes. So you set up bench marks around the job and check them for settlement etc at intervals against permanent bench marks established outside of the site.

 

[BRIS]

In addition I should have added that transferring horizontal control by triangulation is much more accurate the transferring vertical control by level. For accuracy you need a lot more bench marks than you need horizontal control points.