Article by: Mike Barrett on 21Aug2012

This is the first in a series of articles that examines the intricacies of creating and maintaining digital maps by TomTom. This article looks at the Map Geometry, how the roads are discovered and the basemap created.

Essentially the TomTom digital map can be regarded as a series of layers, starting with the basemap defining the geometry of the roads, the next layer provides the map attributes (names, turn restrictions etc), then on top of that comes community contributed content, then we have the 'real road speeds' provided by IQ Routes, and then at the top of the stack is the TomTom HD Traffic.

These can conceptually be viewed as different components drawn on tracing paper and placed on top of each other. Each higher layer provides more information and adds more value to the layers below.

This is also true of what is called the basemap. This starts with the basic lines representing the roads, then the next layer identifies these roads with names etc.

Although we regard a road as a line the digital map contains no lines at all. The core of the map is just a series of points called nodes. The nodes have data associated with them, particularly the GPS co-ordinate to locate it. One of the other critical pieces of data is an association with another node or to put it more simply point A is connected to point B. However it is not as simple as that because point A could also be connected to point C forming a junction. From this we can see that the roads are just conceptual rather than actual. This is not particularly important as far as the end product is concerned, but is critical when you try to create a route. See the section on IQ Routes for more information.

Having summarised what a digital map consists of we now need to understand how these data are collected in the first place. Nowadays it is quite rare that a map will be created from scratch, but it is quite easy to see how it can be done. All you need to do is go out to the location you want to map and start collecting points. It is actually quite easy and anyone can do it just grab your GPS tracker and off you go... This will record a series of points for you and from that you will be able to draw a map.

This is all well and good, but a series of dots is not that useful other than a starting point to create your real map. Typically a tracker will save a point every few metres, but a road could be hundreds of metres long with no intersections so why bother saving all the intermediate points? This is where the TomTom map creation and management application comes to the fore. Called Cartopia it amalgamates a number of different data sources and can provide overlays to assist the operator in determining how to create the map elements.

There are many sources for the initial data, ranging from Local Authorities, National Planning and Mapping agencies (ie Ordnance Survey), driver feedback, MapShare, Satellite Imagery etc. TomTom consolidate 50,000 different Authourative Sources into the map database. All of these can provide information about different aspects of the map data which are combined to produce the map that is delivered to your SatNav device.

So getting back to basics the map geometry is the core component of a digital map. TomTom have a number of different ways to generate this input, the common means are: Local Authorities issuing plans of road changes; mapping van survey; and probe data.

One of the most important sources of new information is the TomTom mapping van. The TomTom Mapping Van is specially kitted out with cameras, Lidar sensors, gyroscopes, redundant computer systems, and of course very accurate Differential GPS. All of these devices record information to redundant hard disks providing georeferenced data to be processed back in the mapping center.

We are all used to using GPS for accurate locations, but there may be times when GPS is not available or reliable. The TomTom mapping van includes sensors on the wheels to determine the distance travelled, and a gyroscope to record the changes in direction of the van. This provides very accurate movement records which when combined with the GPS allows the mapping of underground road networks as well as carparks and other environments where GPS signals are challenged.

Our video below provides an overview of the TomTom mapping van with a quick on-board trip to show the sensors in action.

All of this information is fed into the Cartopia database and allows the operators to get almost as good a view of the surroundings as actually being there. The cameras capture images every 8 metres in the city and 10 metres on highways. With a view of 360 degrees all the signposts, roadside features etc can be viewed back at the editor's workstation. It should be noted that all the images taken are for internal use only so there are no privacy issues involved.

Now with six photos being taken every 8 metres you can calculate that over a stretch of road of 1Km 750 photos are taken. Assuming that a mapping van surveys 200Km a day you suddenly have 150,000 images per van per day to look through. This is obviously not practical so image recognition software has been utilised to pick out certain parts of images for example road signs. The software is programmed to recognise certain patterns such as speed limits. These can be automatically applied to the database.

Armed with this abundance of information the map editor can start working on the map. The basic geometry of the roads can be plotted in Cartopia. Cartopia is a bespoke application specifically written to provide the functionality needed to create and maintain the digital map data. At it's simplest it has basic CAD functions allowing the user to draw a series of lines and provide the ability to connect them together into a network.

To assist the editor in making the correct decisions the computer display can have a number of overlays applied to it. This is a little like adding different information on transparent tracing paper and compositing the sheets so that a view of all the images can be seen. A common base layer would be an aerial or satellite image, on top of this could be the GPS traces from probe vehicles, on top of this the GPS track, and at the top level would be the map details that are being worked on.

Thus it becomes easy to create an accurate map layout even without having a mapping van visit the location. Given a recent aerial photo of the new roads it is easy to draw the nodes and connect the dots, but the rest of the attributes on the road are less easy to determine. This is when the community input of MapShare proves its value. MapShare provides local expert input to correct map data.

TomTom MapShare is the subject of our next article in the series: How TomTom makes maps Part 2 : MapShare.

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