Shapefiles originated as a way of storing objects in an ESRI Geographical Information System (GIS). In addition to storing the 2- or 3-dimensional co-ordinates of map objects, they can also store any number of other attributes about the object, such as the traffic flow data for a road. DXF files cannot do this. However, unlike a DXF file, each shapefile can only contain one type of object, although there may be thousands or even millions of that type of object in the file.
What Information do you need?
If you are using shapefile format, then ideally you will need the following shapefiles:
- Road segments with traffic flows
- Building outlines
- Noise barriers
- Ground contours
- Ground-type outlines
NoiseMap gives you a lot of flexibility about the content of each of these files. The preferred content of each of these files is described in Appendix 1 of the main NoiseMap manual. Very often, some of the information will be missing from the shapefiles that you have been given. There are several options for dealing with this, which are illustrated by the following examples. There are many other possibilities.
Using Default Values
When you import the shapefile into NoiseMap, you can tell NoiseMap what values to assume in place of the missing values (these are called the ‘default values’). For example, the shapefile may not contain information about the type of road surface to use. So you might want to tell NoiseMap to assume a bitumen surface with a 2mm texture depth ‘by default’. It may be possible to add the missing information later on, because you can change the default values to the correct values after you have imported the shapefile. There are various tools to help you to do this.
Calculating Missing Values
If you have a ‘ground model’ (i.e. ground height contours) but the roads shapefile does not have any height data, you can import the ground model first and then tell NoiseMap to get the height of roads from the ground model. Usually, this will mean that all the roads will be at ground level, although if the roads shapefile has information on the height of the road above the ground, then this can be used to get the height of elevated roads as well.
Importing from another Shapefile
If the road shapefile does not have traffic flows in it, you can import the roads and then import the traffic flows from a separate file. In this case, it is essential that the traffic flow reference numbers in the road shapefile and in the traffic flow file are in correspondence, otherwise the correct flows will not be assigned to the correct roads.