# GPS

For digital maps to be useful, roads, lanes, and related objects such as street signs must be accurately geolocated on the earth. The Global Positioning System (GPS), and several other satellite positioning systems, achieve excellent accuracy while taking into account the irregular shape of the earth and the influence of gravity on the rate at which time passes. HERE makes extensive use of GPS for these reasons.

GPS consists of 24+ satellites in orbit and numerous receivers on or near the ground, such as in cars and aircraft. Each satellite knows its current position and time very accurately and transmits this information to receivers. The satellites achieve this accuracy using precise atomic clocks that are synchronized with each other and clocks on the ground.

By measuring the time it takes for signals to travel from a satellite to a receiver, the distance between the two can be measured because the speed of radio waves is precisely known, and the distance is proportional to the time delay in the signal. By doing the same with at least four different satellites (three for position and one to compensate for the error in the receiver's clock relative to the atomic clocks in the satellites), the location of the receiver can be triangulated in 3D space, then translated to useful coordinates -- such as the polar coordinates of latitude, longitude, and elevation relative to mean sea level.

In practice, this process is more complicated than it sounds. Einstein's General Theory of Relativity tells us that time itself flows at different rates depending on the strength of the gravity field at different places. Hence, variations in gravity affect the time it takes for signals to travel from a satellite to a receiver, which in turn affects distance measurements: The gravity field on the surface of the earth is typically stronger than gravity in orbit because satellites in orbit are further from earth's center of mass. Also, the gravity near the surface varies slightly depending on the elevation of the receiver and the density of the earth (which is not uniform) in proximity to the receiver.

Further, the earth is not a perfect sphere. Earth is a slightly irregular ellipsoid, sometimes called an "oblate spheroid" or "3D geoidal ellipsoid," in part because earth is mostly liquid magma and rotating -- a bit like a blob of hot jelly, covered with a thin crust, floating in space and spinning like a top. To be accurate, GPS must take into account these irregularities in earth's shape, as well as the effects of gravity on the flow of time itself. For details on the irregular shape of the earth, see the next section (below).

## 3D geoidal ellipsoid

Earth is a geoidal ellipsoid in space, not a perfect sphere. The 3D ellipsoid shape recognizes that the earth bulges out at the equator and has a major and minor axis, as opposed to a sphere that has the same radius everywhere.

The geoid captures many of the irregularities in earth's shape, as the ellipsoid is not exactly accurate by itself. In the following conceptual figure, the red curve illustrates the geoidal undulation of the earth's surface, relative to the black curve illustrating the ellipsoid.

The geoidal ellipsoid of the earth can be approximated by mathematical projection systems modeling earth's irregular shape. A few projection systems are available. When HERE collects map data, we use the ECEF ITRF2014 projection system at epoch 2017.0. (ECEF stands for Earth-Centered, Earth-Fixed and ITRF stands for International Terrestrial Reference Frame.) Third parties that collect map data and submit it to HERE may use a different projection system. Without the use of a projection system, or consideration of Einsteinian changes in the flow of time, the positional error from measuring transmission time alone would be large (multiple kilometers).

## Contributions from LiDAR and raster photos

HERE vehicles collect LiDAR data as they drive along roads. LiDAR stands for "laser imaging, detection, and ranging." It works by starting from a known location, such as a vehicle using GPS to track its geo-position. Special lasers mounted on that vehicle fire in specific directions. A receiver on that vehicle measures the time for the light that's reflected off nearby objects to be returned, which allows the distrance between the vehicle and surrounding objects to be determined. The result consists of 3D point clouds that are accurately geolocated.

HERE uses LiDAR to identify and geolocate road furnature (e.g., barriers, poles, etc.) and localization objects (e.g., road signs), as well as to automatically model 3D building facades, so our maps more closely resemble real places and support navigation with point-of-view (PoV) cameras. Example:

HERE vehicles also take photos (raster images) as they drive on roads. Usually, these photos are not used directly in HERE mapping products, but they are used indirectly: HERE developers, tools, and contributors can use photos to confirm things like the contents of sign faces and the textures of building facades.