BASICS OF LOCATION DATA

Location data are information about the geographic positions of devices (such as smartphones or tablets) or structures (such as buildings, attractions).

The geographic positions of location data are called Coordinates, they are commonly expressed in Latitude and Longitude format.

Geopositioning, also known as geotracking, geolocalization, geolocating, or geolocation, is the process of identification or estimation of the geographic position (as a noun) of an object.

Locating Places on Earth

Let’s begin by fixing our position on the surface of planet Earth. As we discussed in Observing the Sky: The Birth of Astronomy, Earth’s axis of rotation defines the locations of its North and South Poles and of its equator, halfway between. Two other directions are also defined by Earth’s motions: east is the direction toward which Earth rotates, and west is its opposite. At almost any point on Earth, the four directions—north, south, east, and west—are well defined, despite the fact that our planet is round rather that flat. The only exceptions are exactly at the North and South Poles, where the directions east and west are ambiguous (because points exactly at the poles do not turn).

We can use these ideas to define a system of coordinates attached to our planet. Such a system, like the layout of streets and avenues in Manhattan or Salt Lake City, helps us find where we are or want to go. Coordinates on a sphere, however, are a little more complicated than those on a flat surface. We must define circles on the sphere that play the same role as the rectangular grid that you see on city maps.

Figure 1: Latitude and Longitude of Washington, DC. We use latitude and longitude to find cities like Washington, DC, on a globe. Latitude is the number of degrees north or south of the equator, and longitude is the number of degrees east or west of the Prime Meridian. Washington, DC’s coordinates are 38° N and 77° W.

A great circle is any circle on the surface of a sphere whose center is at the center of the sphere. For example, Earth’s equator is a great circle on Earth’s surface, halfway between the North and South Poles. We can also imagine a series of great circles that pass through both the North and South Poles. Each of this circles is called a meridian; they are each perpendicular to the equator, crossing it at right angles.

Any point on the surface of Earth will have a meridian passing through it (Figure 1). The meridian specifies the east-west location, or longitude, of the place. By international agreement (and it took many meetings for the world’s countries to agree), longitude is defined as the number of degrees of arc along the equator between your meridian and the one passing through Greenwich, England, which has been designated as the Prime Meridian. The longitude of the Prime Meridian is defined as 0°.

Why Greenwich, you might ask? Every country wanted 0° longitude to pass through its own capital. Greenwich, the site of the old Royal Observatory (Figure 2), was selected because it was between continental Europe and the United States, and because it was the site for much of the development of the method to measure longitude at sea. Longitudes are measured either to the east or to the west of the Greenwich meridian from 0° to 180°. As an example, the longitude of the clock-house benchmark of the U.S. Naval Observatory in Washington, DC, is 77.066° W.

At the internationally agreed-upon zero point of longitude at the Royal Observatory Greenwich, tourists can stand and straddle the exact line where longitude “begins.”(credit left: modification of work by “pdbreen”/Flickr; credit right: modification of work by Ben Sutherland)

Your latitude (or north-south location) is the number of degrees of arc you are away from the equator along your meridian. Latitudes are measured either north or south of the equator from 0° to 90°. (The latitude of the equator is 0°.) As an example, the latitude of the previously mentioned Naval Observatory benchmark is 38.921° N. The latitude of the South Pole is 90° S, and the latitude of the North Pole is 90° N.

WGS COORDINATES

The World Geodetic System (WGS) is a standard for use in cartography, geodesy, and satellite navigation including GPS.

The coordinate origin of WGS 84 is meant to be located at the Earth's center of mass; the uncertainty is believed to be less than 2 cm.

What is geocoding?

Geocoding is the process of converting addresses (like "1600 Amphitheatre Parkway, Mountain View, CA") into geographic coordinates (like latitude 37.423021 and longitude -122.083739), which you can use to place markers on a map, or position the map.

Reverse geocoding is the process of converting geographic coordinates into a human-readable address.

Esempio:

https://developers-dot-devsite-v2-prod.appspot.com/maps/documentation/utils/geocoder#q%3D13.664275%252C7.930951

References:

https://developers.google.com/maps/documentation/geocoding/overview

https://www.quadrant.io/resources/location-data

Triangulation

A global positioning system (GPS) device uses data from satellites to locate a specific point on the Earth in a process called trilateration. To trilaterate, a GPS receiver measures the distances to satellites using radio signals. Trilateration is similar to triangulation, which measures angles, depicted in this illustration

The GPS System

The GPS system consists of 3 segments:

1. The space segment, consisting of 24, or more satellites, with accurate atomic clocks on board, continuously transmits ranging signals to the Earth.

2. The control segment, consisting of a number of ground stations, which monitors the satellites, computes their orbits and clock offsets, and uploads this information to the satellites, which in turn encode this information on the ranging signal.

3. The user segment, simply consisting of a GPS receiver, which tracks 4 or more GPS satellites, and computes its own position.

Global Positioning System space segment consists of 32 satellites that are circulating in the medium orbit around earth. Each satellite broadcast specific microwave signals at 1575.42 MHz which GPS receiver positioned on or near Earth surface uses to determine location, speed, movement direction and height above sea level.

GPS satellites are positioned at 20200 km above the sea level on earth and are moving at 3.9km/s. In such way, each satellite circles the Earth twice a day.

How GPS receivers get data from the satellites? Firstly, GPS receiver calculates its position by the messages it gets from GPS satellite. A message contains very precise time of sending the data from the satellite to the earth surface. Each satellite keeps sending messages with precise information on satellite orbit as well as not so precise information on the orbit of other satellites in the system. When the satellite signal is received, receiver determines distance from each satellite and with use of geometry and trigonometry algorithms it calculates exact position on or near Earth surface. Then it converts raw data into a user-friendly form such as graphic or alphanumeric display of latitude and longitude (coordinates).

In order to determine exact 2D position, GPS receiver needs signal from at least 3 satellites. For the exact height above sea level, it requires signal from at least 4 satellites. Since the total number of GPS satellites is 32, more than 4 necessary satellites are available to GPS receiver at any moment. Receiver uses this redundancy in order to determine as accurate position as possible and to avoid atmospheric and reflection influence on GPS signal path.

https://www.tudelft.nl/citg/over-faculteit/afdelingen/geoscience-remote-sensing/education/bsc-education/reader-on-gps-positioning/