Terrain Model

It is recommended that this paragraph is read thoroughly.

Any Novapoint Noise project must be attached to a terrain model. The terrain model is basis for reading a map into AutoCAD, calculating of gradient, collecting height information about buildings and above all generating terrain profiles between the noise sources and the calculation points.

The terrain model has to describe the terrain as accurately as possible. Even small errors and inaccuracies in the terrain model may in some cases get a noticeable impact on the result. Therefore it is quite obligatory to perform a thorough quality control of the terrain model.

In this chapter it is explained what should be prepared in the terrain model and what should be controlled. Users of Novapoint Noise must know this prior to starting noise calculations.

• Compressing Terrain Models
• Feature Codes, Groups, Levels
• Height Error Check
• Buildings

The following geometrical data about the road is to be collected from the terrain model directly or diverted from data collected from the terrain model:

• The centre line of the road, horizontal geometry.
• The height of the road in each noise source.
• The gradient of the road in each noise source.
• The road edges/hard shoulders, placement and height.
• Centre line, horizontal geometry (See Roads).

The height of a road in a noise source is calculated from a terrain profile perpendicular to the centre line in the noise source (same way the terrain level in the centre line is calculated in the Road model). In order for the heights to be correct, the road should be well described in the terrain model. If a road does not have well defined heights connected to the road edge or centre line, the road level will be inaccurate. Such inaccuracy can lead to an incorrect sight line, being too high or too low, and causing errors in the calculation of barrier attenuation.

The gradient of the road is directly included in the calculation result. Steep gradient together with large percentage of heavy vehicles may give considerable contributions to the noise level. The gradient is calculated from the road's height in the relevant noise source and the noise source next to it. If these heights are wrong, we may soon get a wrong gradient as well and by that wrong gradient correction. The gradient adds to the noise value regardless if it is positive or negative.

For roads where the centre line has a given height in the terrain model, corresponding heights will be calculated in Novapoint Noise. All Road model designed roads entered to the terrain model, will have a centre line with height in the terrain model.

Roads which do not have heights in the centre line but have heights on the road edges will mainly be straightforward cases. The heights in the centre line will then in practice be interpolated between the edges of the road. We may have a few inaccuracies in calculation of heights in junction areas and other places where there are “holes” in the edge lines, see the figure below.

The height in the centre line (b) interpolated between point (a) and ©. Point © is the first intersection point between the terrain profile and a line on the upper side of the road. The height in (b) will be different than height © has. With that there will be some uncertainty in the height of point (b).

To avoid such problems a help line in the terrain model may be inserted as shown below:

For roads that neither have heights on centre line nor road edges, but just contour lines e.g. with equidistance 1 m, we really have somehwat little data about the road to get accurate heights on it. This may lead to some uncertainty in the calculation result.

In theory a calculation of the height of the centre line of the road may become as much as 1 m wrong. In practice the discrepancy between calculated height and real height rarely will be that big, but will be between 0 and 1 metre. In the figure below there is shown a case where the height in point (b) is almost 1 m wrong. In this case there will also be a difference of height of 1 metre between two neighbour noise sources. This will also lead to a gradient of 20 % (by a section interval of 5 metres), which leads to a considerable correction in gradient. At a gradient of more than 15 %, one will get an error report in the calculation of gradient. A calculation point lying close to these two noise sources may get relatively large errors in the calculation result, especially if the height of the noise source leads to that we only barely get free sight between calculation point and noise source.

To get a little closer to reality, we recommend that a vertical alignment is created for the centre line, and further that there is created a Road model project to describe the rest of the road. This Road model project may be given side delimitation to the hard shoulders and be saved back to the terrain model.

In the Road model it is possible to get an automatically inserted vertical alignment with a fixed height above the existing terrain. This function is not suitable for this purpose by that very fact that the terrain height is calculated in the same way in the Road model as in Novapoint Noise. With that we still have the same uncertainty in the heights of the noise sources.

The approximate profile numbers may be created by plotting it on a graph paper (profile, height) where the centre line intersects the different contour lines. We then will have a number of points the road is going through. To find the profile numbers where the contour lines are intersecting the centre line, the centre line may be drawn using the Novapoint function “Draw horizontal geometry…”. Further approximate profile numbers ±1m for those intersection points may be read.

In addition to this, any information about a road's crest and sag points should be added, so the road's vertical alignment is as accurately described as possible. The vertical alignment can also be compared with the terrain profile we get from the Road model. If there are large derogations, one should check if something is incorrect.

The placement of the road edges is significant due to the fact that just the terrain profile from road edge to calculation point is included in determining the reflection plane. The height of the road edge in each terrain profile is important to be able to decide whether the road edge shields the calculation point. This is frequently the case when the calculation points are lying lower than the road. 0.5-1 metres height of hard shoulder may in unfortunate cases amount to several dB miscalculation.

The hard shoulders of the road should preferably be lying as continuous lines with separate feature code in the terrain model. We have to “tell” the terrain model which feature codes the lines defining road edge has. Prior to this, there should be a check whether the feature codes we believe is the road edge really define the road edge along the entire distance.

When a terrain profile is generated for Novapoint Noise, the first point on each side of the centre line is defined as the road edge, provided the feature code matches.

With roads in the terrain model that originate from a Novapoint Road Model, road edge definitions is normally not a problem. In many cases feature code 7036 and 7037 are used for the road edges (this must be checked). The lines usually are continuous as well. If the surface description in the Road model is used in a particular way, the road edges may have different feature codes in different places. In such cases, all feature codes must be checked. One should also check if traffic lane lines and road edge lines use the same feature code. If this is the case, traffic lane lines will incorrectly be used as road edges.

In Road model designed roads there will always be heights on the road edges. Therefore we will not have any problems concerning statement of heights on the road edges.

Existing roads where there are heights on the road edges in the terrain model will cause large problems. If “holes” in the road edges (see last paragraph) are not sealed, we may get terrain profiles where road edges are not found or the road edges are placed wrong. The sealing of such “holes” will in other words have consequence for the definition of road edges.

As long as “holes” in the road edges are sealed, we will not have any problems concerning heights on the road edges.

For existing roads where the road edges are without height in the terrain model (height = -999.000), it is recommended to describe the road using the Road model. Centre line and vertical alignment are read to the Road model. Further the Road model module is used to produce a draft for surface and road pavement description. This draft is adapted so that the road reflects the real road as closely as possible. Possible bus stops and walkways should also be inserted. Edge calculation limits are entered for the road edges (the outermost surface in surface group 2 or 3). A profile calculation is run, and the project is saved back to the terrain model.

If it is difficult to adjust the width, the edge lines may be entered as lines as surface edge for surface ±1.1 or ±2.1 (possibly others). In that case 10-tables for the edge lines have to be produced in advance.

We now have a Road model designed road as basis. We collect feature codes for the road edges from the lines in the Road model defining the road edges. The elevation points of the road edges will not be 100% accurate compared to the real heights, but it will be as accurate as it is possible to get it from a map with 1 m equidistance.

The largest uncertainty factor lies in the decision of the vertical alignment. Further there is some uncertainty in the description of the road in the Road model (width and gradient of the surfaces).

This function is described in the Novapoint user's manual.

Feature codes/groups holding building data have to be read in 3D to be registered as buildings in Novapoint Noise (see the chapters Noise drawings and Project files). Generally for Novapoint Noise it is unimportant whether the rest of the map is read in 2D or 3D. Input in 3D has the advantage that we may use the list function in AutoCAD to find the elevation of different objects in the map.

If the map is already in 2D, the following may be done to get the buildings in to AutoCAD correctly:

• Delete the contents of all layers in AutoCAD where there are buildings (all having names ????????05*). This may be done by freezing all other layers in AutoCAD and then use the AutoCAD command erase all.
• Set to passive all feature codes in the terrain model except 5000, 5001, 5002, 5031, 5032, 5082.
• Read in the map again in 3D.
• Get back to the terrain model. Set to active the feature codes that are to be active and passive the feature codes that are to be passive when Novapoint Noise calculations are to be run (The buildings, feature codes in the 5000 series, are to be set to passive).
• Set the correct layer status in AutoCAD.

If two active roads intersect at different levels, this may cause problems within calculations. To make sure that the calculations are correct, feature and group codes for road elements above the road to be calculated, must be switched off. We have made an example attempting to illustrate this:

In a two-level intersection, the road A crosses over road B on a bridge. Make sure that the feature and group code for the bridge is turned off for road B when road B is calculated, while the feature and group code for the bridge must be switched on for road A when road A is calculated. How this is done, depends on how the feature and group codes are defined in the terrain model. If the bridge is “active” in the terrain model, the feature and group codes for the bridge must be switched off for road B, and there is no need to switch on or off any codes for road A. If the bridge is set to “passive” in the terrain model, the feature and group codes for the bridge must be switched on for road A, while there is no need to switched on or off any codes for road B.

1) Two-level intersection, road A lies above road B
2) The two-level intersection seen from above
3) Profile between noise source (s) and calculation point (p)

In the figure we see from the left the two-level intersection in an aerial view, in the middle: plan sketch of the intersection, and to the right: sketch on how the profiles are being apprehended for a noise source on road B from a calculation point p. The last sketch is divided in two: In case I are feature/group codes for the bridge switched on during the calculation, while feature/group codes for the bridge are switched off during calculation in case II.

The selections made in the dialog box Terrain Model Codes overrule feature and group codes from the terrain model for the selected road. All objects within the selected feature/group codes are turned off/on during the calculation for the selected road. Therefore, objects which are to be turned off/on must be moved to a separate feature/group code.