Technical data

Introduction

175

Using Point, Contour, Breakline, and

Boundary Data in Surfaces

Random point data, points taken at a variety of elevations and

coordinates as opposed to interpolated contour data, often makes the

best surface data. To use points for a surface, you can select point

groups or import point files. You can create point groups from the

points in the COGO point database. Point files can be ASCII text files

or Microsoft



Access database files. If you have blocks or lines at

elevations in a drawing, then their coordinates can also be selected as

point data to use in surfaces.

In addition to using points, you can also build surfaces from contour,

breakline, and boundary data. You can have the contours treated as

individual points where the contour vertices are used as surface

points, or you can have the contours treated as breaklines which

prevents triangulation lines from crossing the contours. Surface TIN

lines typically do not cross contour lines.

You usually need to provide more information than only points and

contours to accurately build a surface. For example, TIN lines by

default always connect the points that are closest to each other.

However, if the closest point to another point is across a streambed,

then the elevations that are interpolated from such a TIN line are

inaccurate. To prevent surface triangulation from occurring across

features like roads or streams, you can define breaklines. Breaklines are

constraint lines used by the model that represent abrupt changes in

the surface. TIN lines can be drawn to and from breakline vertices, but

they do not cross the breakline.

NOTE

In Autodesk S8 Civil/Survey products, breaklines were called faults.

By including boundaries in the surface definition, you can control

how the surface extends to its outer limits, and you can mask internal

areas to prevent triangulation from occurring. Boundaries act like

breaklines, but they can either force retriangulation, like regular

breaklines, or they can clip the surface lines that cross the boundary.