Projections and coordinate systems

In cartography, map projections are usually used to transfer the curved surface of the (three-dimensional) earth onto 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 the representation of any area of the earth's surface. Therefore, when combining base maps, it may be necessary to automatically switch a map to another map projection.

As a user of the easymap software you will only occasionally have to deal with coordinates. However, sometimes it is unavoidable, so here is a short overview.

Every point on earth can be described by a coordinate. If points or symbols are to be displayed in easymap, they must also be placed via coordinates. In some cases you specify them explicitly - e.g. if you place the symbols of a site-related analysis via coordinates. In other cases easymap calculates the coordinates internally itself, e.g. as the center of an area. Boundaries of areas or lines are also placed using coordinates. In these cases the line consists of a sequence of points which are connected in a straight line. The support points are also located via coordinates.

Geographic Coordinates

The absolute standard for coordinate data on Earth is geographic coordinates. Every point on the earth's surface can be described using a value pair of geographical latitude and geographical longitude.

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 known as "latitude" or "lat") describes the position in north-south direction, where 0° is the equator, +90° is the north pole and -90° is the south pole. So 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 indicating the geographical longitude, one often finds the additions N for coordinates north of the equator, and S for coordinates south of the equator (e.g. 50° N, 35° S) instead of a positive or negative sign. In electronic data processing, this information is difficult to evaluate and is not supported by easymap. You can convert these coordinates into a usable format by omitting this addition and giving them a correct sign.

Longitude

The geographical longitude (also known as "longitude" or "Lon") describes the position in west-east direction, where 0° is the "central meridian" in Greenwich. From there, positive coordinates count east (e.g. Hamburg is at +10°, Moscow at +37.5°, Tokyo about +140°), while negative coordinates count west (e.g. Lisbon is 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 indicating the geographical longitude, one often finds, instead of a positive or negative sign, the additions E (or E, English "East") for coordinates east of Greenwich, and W for coordinates west of Greenwich (e.g. 10° E, 122° W). In electronic data processing, these indications are difficult to evaluate and are not supported by easymap. You can convert these coordinates into a usable format by omitting this addition and giving them a correct 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.

The earth is in reality not a sphere, but rather a slightly "flattened" sphere, a 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 there are geographic coordinates in another system, you will notice a slight misplacement of your points in easymap.

Other coordinate systems

Projections

An old challenge of cartography is to depict the spherical shape of the earth on a surface. This has not changed even in the computer age, 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 the reproduction, various projection methods (in short "projections") have been developed, which use complicated mathematical procedures to reproduce the spherical shape of the earth on a surface. Almost all printed maps and also easymap internally use such methods 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.

Usually, one does not come into contact with such projection coordinates. Projection coordinates again consist of a value pair 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 geographic coordinate 50°N 10°E corresponds to the coordinate (1113194;6413524) of the Mercator projection.

Use of Projection coordinates

easymap processes in most places only geographic coordinates and usually only shows geographic coordinates.

If your data are available as projection coordinates, you can have them converted back to geographical coordinates using 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

The projection method for the output is determined by easymap itself based on the map base used. Each map base contains the coordinates either in the form of geographic coordinates or as projection coordinates. To ensure a speedy image buildup, these coordinates are then also used for the output.

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

easymap allows the use of map bases that use different coordinate systems in one map. When combining map bases, easymap selects a projection that is valid for the entire area covered.

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

Specification of projections

In some places a coordinate system can be selected in easymap. The easiest way is to use a given system from the list, e.g. "World 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.