A geographic object such a city, a river or a mountain peak only can be located if it is possible to put it in relation to other objects with know positions. It is about determine its location in a coordinates net.
When there is a coordinate system as reference it can be defined a location of any point in the Earth surface.
It is the oldest coordinate system. Each point of Earth surface is located in a intersection of a meridian with a parallel.
Meridians are maximum sphere circles which planes contains the rotation axis or pole axis.
Origin meridian (know as the initial or fundamental) is the one that passes through the ancient British observatory of Greenwich and chosen conventionally as the origin (0º) of longitudes over the Earth surface and basis to time fuse counting.
To East of Greenwich the meridians are measured by increasing values up to + 180º. To the West its coordinates are decreasing up to the minimum limit of – 180º.
The parallels are sphere circles which plan is perpendicular to pole axis. The Ecuator is the parallel that splits Earth in two hemispheres (North and South) and is considered as the origin parallel (0º). From Ecuator to poles direction there are several parallel planes which sizes decrease until become a point in North pole (+ 90º) and South pole (-90º).
A point in the Earth surface is represented by a latitude and longitude values.
Longitude of a place is the angle distance between any point to the initial or origin meridian.
Latitude of a place is the angle distance between any point to the Ecuador line.
Because it is a system that considers angle from Earth center, the geographic coordinate system is not a convenient system for applications to measure distances or areas.
In these cases it is recommended to use a better coordinate system such as planar coordinate system.
Planar coordinate system, known as the cartesian coordinate system, is based in two perpendicular axis, usually the vertical and horizontal axis, which intersection is named origin, established as base to a location of any point in the plane. The origin normally has plan coordinates (0, 0) but it can by convention receive different values (offsets). Thus they can have an origin with coordinates (offset_x, offset_y).
In this coordinate system, a point is represented by two numbers: one corresponding to the projection over the x axis (horizontal), normally associated to the longitude, and the other correspondent to the projection over the y axis (vertical), normally associated to the latitude.
In a GIS the planar coordinates normally represent a cartographic projection and therefore are mathematically related with the geographic coordinates so they can be converted to each other.
Every map is an approximately representation of a Earth surface. They are approximately because the Earth, spherical, is drawn in a plane surface.
A map elaboration consists in a method in which every point on Earth corresponds, in geographic coordinates, to a map point in planar coordinates. To obtain this correspondence it is utilized the cartographic projection systems.
There are different cartographic projections, once there is a variety of ways to project over a plane the geographic objects that characterize a Earth surface. Consequently it becomes necessary to classify them in their several aspects in order to better study them.
In projection classification it is analyzed the cartographic projection systems by the adopted surface and deformation grade.
Regarding the surface type of adopted projection the projections are classified as: planar or azimuth, cylindrical and conic. The projections represents the curve surface of Earth over a plane, a cylinder or a cone tangent or secant to the Earth sphere.
To characterize a datum it is utilized a reference surface positioned in relation to real Earth. It is, therefore, a mathematical model that replaces the real Earth in cartographic applications.
A planimetric or horizontal datum is established from a latitude and longitude of an initial point, from the azimuth of a line that begins from this point and two constants needed to define the reference ellipsoid. Thus, it is prepared a base to calculate the horizontal control values.
There is also a concept of vertical datum referred to heights measured in Earth surface.
Scale is the relation between the element dimensions represented in a map and the corresponding dimensions on the Earth surface.
The scale is an mandatory information to any map and generally is represented in a numeric format.
The numeric scales or fractional are expressed by fractions where denominator represent the natural dimensions and the numerators the reduced dimensions. They are indicated in this format: 1:50,000 or 1/50,000.
The 1:50,000 scale indicates that the measurement unit of the maps corresponds to 50,000 units of the same measurement in over the land. Thus, 1 cm in the map corresponds to 50,000 cm (or 500 m) on the land.