HD maps, or high-definition maps, are maps detailed enough for a car to know which lane it is in. An HD map records the exact position of every lane, lane marking, sign and traffic light, accurate to within about 10 to 20 centimetres, along with the speed limit for each lane and any hazards ahead. HD maps are made for advanced driver assistance systems (ADAS) and for autonomous driving software. They are bought mainly by carmakers in Germany, the United States, Japan, China and South Korea.
An ordinary map for people shows a road as a single line, which is enough to plan a route. An HD map shows the road as a set of lanes. For each lane, it stores the centre line and both edges, the type of marking, the slope and the curve. It also stores the position of each sign, barrier and gantry, and where lanes split or end.
Why does a car need such a map? A car's cameras, radar and lidar see only a few hundred metres ahead. They can also miss a faded lane marking, or a sign hidden behind a lorry. The map tells the car what lies beyond: a sharp bend after a hill, or an exit lane that starts in 800 metres. The car also uses the map to find its exact position. Satellite positioning such as GPS is accurate only to a few metres, which cannot tell one lane from the next. So the car matches the lane markings and signs that its cameras see with the ones on the map. This is called localisation.
Imagine a car from a German carmaker on a motorway near Munich, in Germany, at 120 km/h, with its lane-keeping assistant switched on. Two kilometres ahead, roadworks narrow three lanes to two, and the speed limit drops to 80 km/h. The cameras cannot see that far. But the HD map, updated the night before, already holds the roadworks, the new lane layout and the lower limit. At 1 km before the roadworks, the car warns the driver. At about 500 metres, it slows to 80 km/h and steers into one of the two remaining lanes. If the map and the cameras disagree, the car hands control back to the driver.
How is the map kept current? Map makers once sent survey cars to drive every road. Now many updates come from ordinary cars. Their cameras report changes, such as new roadworks, and geospatial AI checks the reports and edits the map. HD maps are also loaded into autonomous vehicle simulation, so that driving software can be tested on virtual copies of real roads. A robotaxi usually drives only inside an area that has been mapped in this detail.
In India, HD maps are at an early stage. Many roads have faded or no lane markings, and road layouts change often, so most Indian driver assistance relies on the car's own cameras and radar. HD maps are a detailed form of the digital map.