When I want to "Simplifying a Toposurface",what is the "surface accuracy"?

[mike99]

what is the meaning of the value that after the name "surface accuracy"?

it is a default setting of 3 inchs?

thanks a lot

[Steve_Stafford]

There is an older thread (http://forums.augi.com/showthread.php?12226-Sub-region-causing-contours-to-shift&p=72528#post72528) that discusses this concept to some degree.


Revit draws contour lines after it triangulates the toposurface. It finds a constant height path across triangles and draws the contour. contour segments When a sub-region is created, Revit re-triangulates so that triangles don't cross the boundary. Since this causes some shift in the triangles, it can also cause some shift in the position of contour lines. The effect will be most noticeable for gentle slopes where a small vertical shift can result in a larger horizontal one. In the surface itself, the change in elevation of any point on the surface due to re-triangulation is never more than an inch or two (2-5cm).


Units selection does not affect shifting of contour lines when making subregions. There are two factors that influence the distance of such shifts: (A) slope and (B) size of topo surface triangles before the split. Smaller slopes result in greater shifts. On the other hand the smaller the size of triangles the less contours will shift. This is all understandable because the precision of original surface model is limited by the number of points that define it.

The fact that contours shift a noticeable distance is not necessarily an indication of a problem. For example if original topo surface is defined by just a dozen of points but covers an area of a square kilometer and varies in elevation by only several centimeters then contours may shift a great deal. The drawings showing contours on subdivided top surfaces should be OK for as long as the inaccuracy of result is less than imprecision of the input and imprecision of the grading bulldozer blade. Nonetheless if you do want to make contours shift smaller distances then you need to increase the number of points that define topo surface especially in the neighborhood of split boundary.


Simplify is attempting to reduce the number of points necessary to generate the surface, changing the value is either increasing or decreasing the vertical offset between points that govern the simplification. It will almost certainly alter the surface so contours don't match a survey as closely.

[mike99]

yes! thanks for your time!!!