Terrain Modification

Pad Template

2D Polyline Closed Perimeter: The most basic use of pad template is for the creation of flat-bottomed pits and flat "building pads" at any desired elevation. The procedure is simply to draw a closed polyline at 0 elevation, then select Pad Template (top command in the Site menu), and enter the pad elevation and side slopes. Here are two examples:

This piece of the larger drawing was carved out by first doing the RECTANG command and making a rectangular polyline as shown above. Then we did Break by Closed Polyline under the Modify menu (using default answers), followed by Wblock to write out the separate drawing. The 2 polylines at right are drawn by the RECTANG command (left) and PL command at 0 elevation.

Draw 3D Polyline: This is an obvious tool for creating terrains. We recommend use of SurvCADD's Draw 3D Polyline at the top of the 3Dpoly pulldown menu. Our first exercise is to build a ramp from north to south into the pit. We will "arbitrarily" start at elevation 1978 by "snapping" to the 1978 contour with the "nea" object snap (nearest), then snap to the base of the pit at 1950 with the nearest snap. Then we will offset this 3D Polyline, connect its ends by Join Nearest, and do a pad template at 2:1 sideslopes. As an exercise, try Viewpoint 3D under the View menu, and choose a SW viewing angle at 35 degrees above the XY plane to get a view similar to this:

In this view, erase the original pad base polyline which are still at 0 elevation. This will prevent the "nea" object snap from finding the 0 elevation base polyline instead of the new, green, pad polyline. Enter Plan at the command prompt followed by 2 Enters to go back to Plan View.

Offset 3D Polyline: After the 3D Polyline is drawn as shown above, do the command Bearing and 3D Distance, "Pline" option, to check the percent slope of the 3D polyline. At approximately 10% grade, it should be "drivable" by haul trucks. Move the 2nd 3D Polyline to contact the base of pit.

Join Nearest: Now find Join Nearest under the Modify menu, and select "Directly Connect" and enter a 35 offset tolerance. Pick both 3D polylines to join them, the result is shown above.

3D Polyline Closed Perimeter: Now we have a 3D polyline closed perimeter which can act as a pad. Within the Pad Template routine, we will no longer be asked for a pad elevation, since the program will obtain the variable pad elevation from the vertices of the 3D polyline. It is advisable to first do a List command under Inq-Set to verify that each pad vertex is non-zero. The 2:1 cut sideslopes (fill sideslope is irrelevant, since we are in cut) leads to the drawing below:

Note that we are consistently using the Pad Template option to Trim existing contours and 3D polylines, and to not retain the trimmed portion. We are also consistently selecting not to draw contours. In this manner, we iterate our way to the desired final terrain. We should also note that we are using the standard 50x50 "number" of cells in the Pad Template routine, windowing the entire site each time. More cells or smaller dimensioned cells leads to a finer calculation.

One-Sided Pad Template: Pad Template also works with a 2D or 3D polyline that is open and not closed. In this case, the routine will ask for which side to offset (with a closed polyline, it always offsets outward). For example, suppose you were concerned where a pit located along the northeast side of the site would "catch" if it sloped at 2:1 from an elevation varying from 1980 at the north end to 1985 at the south end.

To put this 3D Polyline in, select Draw 3D Polyline under the 3Dpoly menu, and choose the prompt for elevation option. Enter the north elevation as 1980 and the south elevation as 1985. The do Pad Template and choose the left side for the offset at 10:1.

The top of the cut did not impact the building pad or parking lot at right.

Bench Pond

Fully "Incised" Pond: The Bench Pond Routine is found under the Site menu. It is based on the use of a closed polyline representing the top of the pond. The program then cuts a circular pond into existing terrain. Unlike Pad Template, the Bench Pond routine works "inward". The main thing to remember is that if you have roughly 20 feet to the center of the pond and you want to go downward at 2:1, do not ask for a depth greater than 10 feet. This will cause one side to pass beyond the other, and you will "hourglass" the interior. Another concept to remember is that the program will cut downward from the drawn polyline which is placed at an elevation representing the water level or top of pond. A separate cut and fill ratio can be applied to the outside of the drawn polyline. If you place the pond fully in cut (fully incised), then one cut ratio would apply to the interior going down to the base of pond, and another would apply to the exterior going up to "daylight". Of course, the same ratio can be entered for both slopes.

Here we have drawn a closed polyline in the lower right of the drawing, and will set its elevation to the lowest elevation the line crosses (as prompted by the program). This will ensure that the pond is fully incised and does not have any fill slopes.

Partial Fill-Partial Cut Bench Pond: A typical farm pond might have the downhill side in fill and the uphill side in cut. In fill, the flat-topped "bench" might be 10 feet. In cut, the bench would disappear. Cut above water into original ground might be 6:1. Cut below water might be 3:1. Fill could be set at 6:1 and 3: below water. In this case, we would remove the top bench in cut. Shown below is a bench pond cut into the top center of the drawing at elevation 1994.

Revisit Pad Template by Doing a Diversion Ditch: If you are getting the idea-try this on your own: Draw a 3D Polyline that will drain the lower right pond into the upper left pond. Do this through use of the Draw 3D Polyline command. Issue the command, and do a "nea" snap maybe one-third down the northwest "slope direction line" in the pond at lower right. Connect with a "nea" snap to a point halfway down the southeast running "slope direction line" in the pond at upper left. List your 3D polyline to be sure it runs downhill from approximately 1992 to 1989, or thereabouts. We're after the concept here. Then do Offset 3D polyline 4 feet either way for base of ditch. Connect the ends up with Join Nearest, tolerating 5' separation. Then Pad Template at 3:1 side slopes in Cut and Fill. Your result is shown below in 3D.

Valley Pond

Constructing a Valley Dam: We can "carve out" another portion of our base map by first drawing a "RECTANG" and then using Break by Closed Polyline under the Modify menu.

We follow this with Wblock and write out a new drawing called Valley1.dwg, seen below.

Unlike "Bench Pond", the "Valley Pond" routine requires only a polyline axis line for the center of the dam. The polyline can be a 2-point polyline or can have several vertices along its length to create a concave or convex dam structure. The main thing is to "overdraw" the axis polyline-make it ride up on the left and right hillside well beyond the desired top of dam elevation. This allows the routine to look inward and find the extents of the dam on each hillside without doing an artificial extension of the polyline. Just "overdo" the length of the axis line and you are in business. Another aspect to concentrate on is the desirability to select enough terrain upstream to enable the program to compute the full waterline extents-the limits of the dammed-up water. Without enough upstream terrain in the initial selection set (which acts like a crossing selection), you will not be able to compute the limits of the water surface and the pond stage-storage information. Our axis polyline runs from approximately 1960 on one side of the valley to 1960 on the other. It crosses the valley at 1931. Let's decide to put the top of dam at 1950 even. We will make the dam 20' wide, with 3:1 downstream and 4:1 upstream slopes. Source of surface model is, as always in this case study, the screen. Say N to Cut Pond Interior and N to Contour. Just follow the prompts in the Valley Pond routine to obtain the drawing below:

3D Polyline by Slope on Surface: How would you start at the top of dam (elevation 1950) and build a road running downhill at 6% grade? Or in general, how would you obtain 3D polylines for roads and diversion ditches that follow the terrain at prescribed grades starting at desired points? The answer is 3D Polyline by Slope on Surface. This routine requires that we make a grid file for the terrain. Use the command "Make 3D Grid" under the DTM pulldown menu, and select a 20x20 cell dimension. Store the file as Dam.grd. Then run the 3D Polyline by Slope on Surface (located in the 3D Poly menu). Pick a starting point on the north side of the dam. Facing downhill, you will go downhill to the right.

After the new 3D polyline is drawn, offset it into the hill with the 3D Polyline Offset command, then join the ends with Join Nearest, then use Pad Template to carve our road into the terrain. Follow that procedure and you obtain the drawing below:

In the drawing at right, we now have a "pad" for pad template. Because the southern side of the pad follows closed the original ground, it may "cantilever" over into the "air" in a few places based on the resolution of the calculation. It is recommended that the fill ratio used to catch the ground be low, such as 1:1 rather than 2:1, so that short cantilevered sections of the pad, if placed on natural 2:1 terrain, don't "skim" over the ground and create unnecessary fill.  For the cut sections, we will use 3:1 to carve the road into the solid hillside. Here is the result in 3D:

2D to 3D Polyline by Surface Model (Drape Command): The 3D view above reveals a 3D polyline running up the base of the stream channel in which the pond was built. Such 3D polylines are important in modeling accurate surfaces for pond design, pad templates and volumes in general. For example, if you were to triangulate and contour the valley at 1' interval (currently the contours are at 4' interval), you would obtain poor valley contours which "square off" in the valley-if you did not select the 3D Polyline "break line". Thus, an important strategy-better yet, policy-is to dress up raw contour maps with valley and ridgeline 3D polylines that act as break lines and restore the true character of the terrain. The best way to make these 3D polylines is to draw 2D polylines in drains and ridges (see the three circled examples below) and "drape" them on the terrain. This is done by the 2D to 3D Polyline by Surface Model command found under the 3D Polyline menu.

Creative Uses of 2D to 3D Polyline by Surface Model: More than a command to "dress up" contour maps for greater modeling accuracy, the "drape" command has unexpected uses.

In the example below, a strata angling along a pit face creates instability in the upper part of the pit. The goal is to "lay back" the pit at 2:1 above the strata demarcation line, and retain the 1:1 slope below that line (which is drawn as a 2D polyline at the outset).

After the 3D Polyline is "draped" on the surface, you should use Change Elevation under Modify to drop the entire 3D Polyline just a little, such as -0.2. This will ensure it is fully in Cut. Then the Pad Template command can be used to perform a one-sided offset at 2:1 in Cut, resulting in the drawing below, shown in 3D view:

Build a Diversion Ditch around the Dam Using Input-Edit Profile and Profile to 3D Polyline: With all object snaps turned off, draw a simple 2D polyline ("PL" command or SurvCADD's "2DP") that starts on the water side of the valley dam and curves around the hill to the south and into the drainage below the dam, as shown below left. The syntax for this using the PL command is Pick first point, pick 2nd point to get a tangent (straight section) going, then to A for arc, arc it around the dam, then do L for line and a second L for length and pick a point that comes off tangent from the arc and ends in the streambed below the dam. Then Enter to exit the PL command.

Now we are going to make this 2D polyline a 3D polyline with a prescribed profile. To prepare for this, we do Polyline Info under the Inq-Set menu and write down the length of the polyline (this will become the length of the profile we enter). This one here is 377.6 feet, which we will round up to 378. We then use List Elevation under the Contour menu and pick on the 3D polyline as close as we can to the point where our 2D polyline makes contact. That elevation is reported as 1929.4 (along with all vertice elevations). We determine ahead of time that we want the first 50 feet of the diversion ditch to be a 1% downhill slope, starting at elevation 1946 (allowing 4 feet of freeboard to the top of the dam at 1950). We are ready for Input-Edit Profile in the Section-Profile menu. We fill out the profile dialog as shown below:

The order of entry might be 0 and 1946 on the first line, 378 and 1929.4 on the third line, and 50 and slope% of -1 on the second line, completing the profile. Save it and Quit from the dialog. The one remaining step to get a 3D polyline is to select the command Profile to 3D Polyline under the Profile menu.  Pick the ditch centerline and apply the newly saved profile to it. Erase the old centerline and you obtain a "yellow" colored 3D polyline centerline.

Use the command Offset 3D Polyline to offset the ditch into the hill 8 feet for an 8' base (or offset 4' either way and erase the original centerline). In either case, you have two parallel 3D polylines. Now do Join Nearest, tolerate 9' of separation and Directly Connect the Endpoints within Join Nearest. (Note how create 3D Polyline, Offset 3D Polyline and Join Nearest become a familiar sequence in doing pad template work!). Now do Pad Template at a cut slope of 1.5 to 1.

If it is apparent that the diversion ditch was initially drawn too far into the pond area and created fill, as seen at upper right, then there is the option to re-design or simply to trim of f the fill portion. This likewise applies to the portion of the diversion ditch at its terminus downstream. Note that gaps in contours that need to be re-closed can be very quickly fixed with Join Nearest. With the trimming completed, the final design appears below in 3D:

Build a Curving Road from Top of Dam to Base of Pit using Input-Edit Profile and Profile to 3D Polyline: Looking at the 3D view above, lets extend the road from the top of dam into the base of the pit, following a uniform grade and entering the pit at right angles on the south side (or right-side as seen above). This "word problem" is nothing more than another iteration of Input-Edit Profile and Profile to 3D Polyline. The first challenge is to draw the 2D polyline using the PL command. We've already learned how to do this and use the second L approach to come off tangent from an arc. We end up with a centerline that might look like the one below:

Just as before, find the length of the polyline using Polyline Info under Inq-Set. We know the profile: it runs from 1950 (the top of dam) down to 1930 in the base of pit. So we go straight to Input-Edit Profile, perhaps name it Road (Road.pro) and put in a simple two-entry profile as shown in the dialog box below:

The slope of the road at -3.17 percent is very acceptable for any type of vehicle. So now we choose Profile to 3D Polyline and apply Road.pro to the road centerline. This time we will offset the new 3D polyline 10 feet either side using 3D Polyline Offset, and erase the original middle centerline (or render it harmless for terrain modeling by the command 3D to 2D under Modify). This raises an important point: when we select entities from the screen using Pad Template, Valley Pond and Bench Pond, the pad or pond polyline itself is filtered out of the selection set and is not used for modeling the original surface.

If we left intact a 3D polyline centerline that was not part of our pad, it would skew the surface model badly. Thus if we offset left and right for the outside of the road, the middle of the road must go, must be erased, unless we chose the "grid file" option for the original surface. After offsetting 10' either side and erasing the center 3D polyline, we Join Nearest at a 21' tolerance. Then we do Pad Template at standard 2:1 slopes. Here is the final 3D view. (You might use Pad Template to divert the ditch and not flood the pit!).

Pad Template in Combination with Design Template for Roads, Ditches and Levees

Roads with Ditches and Berms: The limitation of Offset 3D Polyline within Pad Template is that roads cannot easily be given dynamic characteristics: They won't automatically carve in ditches in cut, or build a berm in fill where fill exceeds a certain threshold, for example. Since these "intelligent" features exist within the Design Template routine in the Section-Profile module, all we need to do is invoke the "template" option within the Pad Template. Pad Template will then go out and get a Design Template and apply it to the one-sided or closed pad perimeter. To illustrate this, let's first make a centerline that will have cut and fill. Consider the centerline drawn below:

If we go a uniform grade from the existing road upward to the ending elevation (which might represent a mining bench), we are "bound" to get cut as we cross the lower point and fill as we cross the second drain. To "guarantee" fill, we will go uphill early and lesson the last portion of the grade, using a vertical curve length of 500 between profile grades. This is another exercise of Polyline Info (for length), verifying start and end grades, and Input-Edit Profile (shown in dialog):

With the profile input (named road1.pro), we can select Profile to 3D Polyline and turn yet another 2D polyline into a 3D polyline. The difference now is that we will not offset the 3D polyline to create a closed, looping "pad". We will instead use a template made in Design Template, and apply it to the 3D polyline.

Entering a Template in Design Template: Design Template, located under the Design menu in the Section-Profile menu, is icon driven. Below is a selection of dialogs that illustrate our entry of a 24' wide road (12' either side) at 2% slope, no shoulder, no subgrade, a 3:1 slope for 6' to the base of ditch, a conditional cut upslope, a 3:1 slope in fill, but a 2:1 berm in fill, rising 4 feet, used when fill exceeds 5 feet.

Note that when you are putting in a ditch, you need to click "Start" at the lower left