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What is a Surface?





A Surface is a model of a physical surface like the original ground at a site, a borrow pit or a road. It starts with TOPO points who's elevations are on the physical surface. From these, TPC generates a TIN (Triangulated Irregular Network) which is a series of contiguous triangles that represent (model) the physical surface. These triangles are sometimes called '3D Faces' or 'Polygon Meshes'.

A Traverse PC surface is composed of the following:

Name and description - each surface must have a unique name that identifies the surface from all the other surfaces in a survey.

Survey points - only survey points with their 'Topo' status turned on are included in a surface.

An optional source traverse - only the survey points in the traverse are used in the surface.

An optional border - the border constrains the surface.

Breaklines - breaklines change the configuration of the triangles in the TIN. We generally think of a breakline as any place two separate areas (planes) within the surface form a line.

TIN - surfaces are created by triangulating the topo points. These triangles are typically discarded once the contour lines are created but can be retaind for volume computations and slope analysis.

Elevation limits - you can limit a surface to a minimum and/or maximum elevation. This is a great way to eliminate elevation blunders but can also help with simple excavation projects.

Surface settings - these are the user settings that determine how to draw the contour lines, shade the slopes, show the breaklines and what layers to put them on.

Surfaces are Dynamic

A surface is generated from survey points. If the coordinates or elevation of a survey point in the surface change, the surface changes. It's pretty simple. So go ahead and create the surface. Later on when you update the survey points to the correct benchmark elevation the whole surface recomputes - dynamically.

You can also select a traverse as the source of a surface. Start your surface with the points you have then go back and 'fill in' the areas that need more detail simply by adding those points to the source traverse. The next time the surface is recomputed the new points are added into the surface - dynamically.

This whole idea of dynamic surfaces will change the way you think about working with surfaces in your surveys - for the better. See Editing Surfaces and Creating Surfaces.

Topo Points

Surfaces only use survey points designated as Topo Points. You can designate Topo Points in a number of different ways. Once the surface is drawn, you can even turn off the topo designation for individual points to fix blunders in your surface. To learn more about editing topo points see Editing Topo Points.


TPC generates its surface from a TIN (Triangulated Irregular Network). It creates triangles that model the surface, connecting all the border and topo points then interpolating the elevations along the edges of each triangle. See Creating a TIN.


Contours are lines of constant elevation that help us visualize the surface. They are generated from the TIN, so the way to edit contours is to edit the TIN.


Breaklines allow you to model grade breaks like ‘top of slope’ and ‘toe of slope’ with a minimal number of points. You can accomplish the same final surface shape without breaklines, but it requires taking numerous points along each breakline. It takes a little practice to gain experience collecting and using breaklines, but the investment is well worth it. See Using Breaklines.


Exclusions are areas within a surface were you do now want TPC to draw the contour lines. You create exclusions by tell TPC to make certain TIN triangles invisible. This is the same convention supported by LandXML surfaces and CAD polyfacemesh entities.


Every surface has a border which constrains the surface. If you think of the surface as a TIN, the border is all the outside edges of all the outside triangles. These are the triangle edges that have just one triangle to the inside and no triangles to the outside.

The border defines the limits of a surface. Only the topo points that lie within the border are used to generate the surface. In this way, the border also affects the shape or 'outline' of the surface.

By default, a surface is constrained to its 'convex border'. Think of this a stretching a rubber-band around all the topo points in the surface. If you don't specify a border for a surface, it defaults to this convex border. This makes creating a surface very simple. See Creating Surfaces.

You can also select a traverse to defines the border of the surface either when you create the surface or anytime you edit the surface. See Surface Borders.


By default, TPC groups a surfaces drawing objects onto layers. Major contour lines and labels are placed on the TPCContourMajor layer and so on. If you want to change the layers that a surface is placed on, use the Advanced dialog. See Surface Layers.

Invisible Faces

TPC supports invisible faces imported from a LandXML surface.

By default, TPC hides the faces that are marked as invisible in the LandXML surface. However, you can turn invisible faces on via the Surfaces TIN dialog.

Restrict Elevations

You can limit the topo points that are included in a surface by elevation. If you have some 'wacky' elevation points in your survey, this is a great way to exclude them from the surface. See Surface dialog.

Surface Settings

Each surface has user settings that determine how the surface is represented in a drawing. These Surface Settings allow you to include major and minor contour lines with the colors, line types, labels and contour intervals you want. Other Surface Settings allow you to draw the TIN lines and breaklines, use shading to delineate the cut and fill areas or color in all the areas with a slope greater than 30 percent.

Each surface has its own settings (the default settings) which allows the surface to be displayed the same way in any number of drawings. But when you need it, a surface can be displayed in a drawing using unique Surface Settings. The unique surface settings are used instead of the surfaces default settings, allowing you to change contour intervals, colors, line types or whatever.

Surface Information

TPC collects information about the contours it creates to help you correct problems or improve the shape of the surface. You'll find information about the surface in the Surface dialog, Volume dialog and Slope Analysis dialog.

Right click any contour line and choose Surface Tools, Surface Info.

Creating Transects (Profiles)

TPC can generate transects from a surface. You just draw a line across a surface where you want the transect to be, then use the Create Transect from Surface command and TPC will create a traverse with a point for each intersection of the TIN and the line, arc or spiral. See Creating Transects From Surfaces.

Generating Cut Sheets

TPC can generate a cut sheet that shows the cut or fill from the surface to each point in a traverse. You might create a traverse for a building pad, translate it to its design grade, and then generate the cut sheet. Now you will know how much to excavate at each building corner to get to grade. See Creating Cut Sheets from Surfaces. (Professional Edition only)

Computing Volumes

TPC generates surface volumes in the Volume tab of the surface settings. You can generate volumes to the surface border (default), to an elevation, or to another surface. (Professional Edition only)

Analyzing Slopes

The Professional edition of TPC Desktop includes tools to analyze surface slopes. TPC can determine the parts of the surface that lie within any arbitrary slope range you specify, such as 0% to 10%, or 11% to 20%. These areas can also be shaded to indicate their slope range. See Slope Analysis.

Adding a Contour Line to a Legend

Right click a contour line and choose Append to Legend. TPC generates a legend item with the contour line type, the contour elevation and the name of the surface.

Optimizing Surfaces

Surfaces can be optimized to improve drawing speed, reduce memory requirements or allow interactive editing. See Optimizing Surfaces.

Moving Survey Point Elevations to a Surface

You can use the Move Elevations to Surface command to move existing survey points to a surface.  For example, you might create a grid of points to stakeout a finished grade surface you created. Just create the points, then move them to the Finished Grade surface.

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