Projections and coordinate systems

In cartography, map projections are usually used to transfer the curved surface of the (three-dimensional) earth to a flat, two-dimensional map. Accordingly, the coordinates stored in a digital map are usually subject to projection. Not all map projections are suitable for displaying any area of the earth's surface. It may therefore be necessary to automatically switch from one map to another when combining base map.

As a user of the EasyMap software, you will only occasionally have to deal with coordinates. Sometimes, however, it is unavoidable, so here is a brief overview.

Every point on earth can be described by a coordinate. If points or symbols are to be displayed in a map, they must also be placed via coordinates. In some cases, you may want to specify them explicitly - for example, if you place the symbols of a location-based analysis using coordinates. In other cases EasyMap calculates the coordinates internally, e.g. as the center of an area. Also area boundaries or lines are placed by coordinates. The line is a series of nodes linearly connected. The nodes are also located by coordinates.

Geographic Coordinates

Geographic coordinates are the absolute standard for coordinates on Earth. A pair of values geographic latitude and geographic longitude can be used to describe any point on the Earth's surface.

The latitude and longitude finally represent the angular distance of any location from the center of the Earth and they are measured in degrees (°) and minutes ('). You can specify fractions of degrees either decimal (e.g. 51.5°) or traditional in degrees, minutes and seconds (e.g. 51°30'00''). Like the time, one degree is divided into 60 minutes and one minute into 60 seconds.

Latitude

The geographical latitude (also called "Latitude" or short "Lat") describes the position in North-South direction, where 0° is the equator, +90° the North Pole and -90° the South Pole. Positive values are on the northern hemisphere of the earth, negative values on the southern hemisphere.

The values must therefore always lie between -90° and +90°, otherwise the coordinate is invalid and cannot be converted.

When specifying the latitude, the additions N for coordinates north of the equator and S for coordinates south of the equator are often used instead of a positive or negative sign (e.g. 50° N, 35° S). These additions are difficult to evaluate in data processing and are not supported in EasyMap. You can convert the coordinates to a usable format simply by omitting this addition and using a plus/minus sign.

Longitude

The geographic longitude (Longitude) describes the position in West-east direction, where 0° is the "central meridian" in Greenwich. From there positive coordinates count to the east (e.g. Hamburg at +10°, Moscow at +37.5°, Tokyo at about +140°), while negative coordinates count from there to the west (e.g. Lisbon at -9°, Rio de Janeiro at -43°, San Francisco at -122°).

There are usually no coordinates beyond -180° or +180°. Due to the fact that the earth is round, however, coordinates beyond this limit are used from time to time, but correspond to a coordinate within the given range. For example, +200° is the same as -160°.

When specifying the longitude, the additions E for coordinates to the east of Greenwich and W for coordinates to the west of Greenwich (e.g. 10° E, 122° W) are often used instead of a positive or negative sign. These additions are difficult to evaluate in data processing and are not supported in EasyMap. You can convert the coordinates to a usable format simply by omitting this addition and using a plus/minus sign.

WGS84

It is not enough to just use the geographic coordinate. The geographic coordinates of a location on Earth vary depending on the figure of the Earth.

Earth is not really a sphere, but an irregular shape approximating an so-called ellipsoid.

Fortunately, there is an absolute standard model for describing the Earth, the WGS84-ellipsoid. Practically all geographical coordinates are implicitly related to this model, one should speak more exactly of geographical coordinates related to WGS84.

If you have geographical coordinates in another system, you will notice this by a small incorrect placement of your locations in EasyMap.

Other coordinate systems

Projections

An old cartography challenge is to represent the spherical shape of the earth on a plane. This hasn't changed in the age of computers, EasyMap is also facing this task.

Geographical coordinates are rarely simply mapped 1:1 to the pixels of the screen (this is called "flat map" or "rectangular projection"). The problem is that one degree of latitude anywhere in the world is about 111 km, while one degree of longitude is shorter depending on the distance from the equator (in Germany, one degree of longitude is only about 70 km). In this case, this makes maps appear rather distorted.

In order to improve display, various projection methods (briefly "projections") have been developed which use complicated mathematical methods to reproduce the spherical shape of the earth onto a plane. Almost all printed maps and also EasyMap use such internal procedures to display the map.

Projection coordinates

You can transform any geographic coordinate to a new coordinate using one of the projection methods. This coordinate is then used to display the location on the screen.

As a rule one does not come into contact with such projection coordinates. Projection coordinates also consist of a pair of values Right value (x) and High value (y) and are usually recognizable by very large numerical values, since distances in meters from the equator or the central meridian in Greenwich are usually used.

Note: The geographical coordinate 50°N 10°O corresponds to the coordinate (1113194;6413524) of the Mercator projection.

Use of Projection coordinates

EasyMap works only with geographic coordinates in almost all places and usually provides only geographic coordinates.

If your data are available as projection coordinates, you can have them calculated back to geographical coordinates via the calculated column coordinate transformation.

You also have the option of displaying the mouse position in the map in projection coordinates as an alternative. This can be set via Options (menu Extras).

Output projection

EasyMap determines automatically the projection method for the screen based on the used map base. Each map base stores the coordinates either as geographic coordinates or as projection coordinates. The coordinates are used for the output to ensure a fast screen layout.

You can see and change the used projection in the Properties of the Map (see Common, Projection).

You can use map bases with different coordinate systems on a single map. EasyMap selects a projection that is valid for the entire covered area.

In individual cases EasyMap cannot determine the coordinate system of a map basis. Such maps can also be used, but numerous functions are then not available (e.g. location-related analyses with coordinate placement).

Specification of projections

It is possible to select a coordinate system in EasyMap in certain situations. You can use a predefined system from the list, e.g. "Welt Mercator (EPSG: 3395)".

If the required coordinate system is not listed, you can also use many systems with a EPSG-Code (Unique numbering system for specifying coordinate systems, introduced by the European Petroleum Survey Group "EPSG"). You must know the EPSG code of the coordinate system or refer to http://www.spatialreference.org/. Then you can enter the value EPSG:(Code) in the field.

Note: You want to use the UTM Zone 34N coordinate system. This is not listed in the predefined systems. Using the search at http://www.spatialreference.org you can determine the EPSG code 2079. You can now enter "EPSG:2079" in the field to use this system.