Projections
Coordinate Systems and Coordinate Reference Systems[edit]
Working with Geographic Information Systems (GIS) requires a consistent method for identifying the precise location of objects on Earth. This is accomplished through the use of a coordinate system, which assigns numeric values to describe a point’s position. All spatial data are defined within such a system. A coordinate system typically includes both horizontal and vertical components, referenced from a defined origin. The horizontal component specifies a location’s position across the Earth’s surface, describing distances east–west and north–south relative to the origin. The vertical component specifies the position above or below a reference surface—often sea level—indicating elevation or depth. A Coordinate Reference System (CRS) builds on this concept by defining how two- or three-dimensional spatial data correspond to real-world locations. A CRS is based on a mathematical model of the Earth, called a datum, and often includes a map projection—a systematic transformation of the Earth’s curved surface to a flat plane. CRSs can be geographic, which use latitude and longitude on an ellipsoidal model of the Earth, or projected, which apply mathematical formulas to produce a planar representation more suitable for mapping and spatial analysis (further explained below). The choice of CRS is critical: it ensures positional accuracy, allows integration of datasets from different sources, and controls the type and magnitude of distortions in distance, area, and direction.
However, because the Earth is roughly spherical, it cannot be represented on a flat surface without distortion. This challenge is addressed through projection systems. A projection system uses defined mathematical transformations to represent the Earth’s surface on a two-dimensional plane. Different projections are optimized for different purposes—for example, preserving shape, area, or distances—and selecting the right projection is essential for accurate spatial analysis.
Types of coordinate systems:
There are various types of projection systems, some global, such as, Geographic coordinate system, projected coordinates system, universal transverse Mercator system, and some local such as., State Plane Coordinate System (USA), India Grid System (Indian Geodetic Datum) etc.
Two most widely used types of coordinate systems are:
- Geographic Coordinate System
- Projected Coordinate System
Geographic Coordinate System (GCS):
Location on the Earth surface is specified by using the Latitude and Longitude which specifies the angle between the point of interest and equator and angle between point of interest and prime meridian. A GCS consists of prime meridian (Greenwich, England), datum (Earth surface or precisely ellipsoid) and unit of measurement (degrees). The most widely used GCS is World Geodetic System (WGS 1984). The system utilizes the location shared by Global Positioning System (GPS).
Projected Coordinate System:
To understand Projected coordinate systems, we must first understand what a projection is. Let us assume that Earth is a massive sized orange. If you try to lie the orange flat on a surface, it is nearly impossible. But, when you peel the orange, the peel of the orange can now be laid down on a flat surface looking something like Figure 2.
The shape of the peel varies based on the way we peel it, and so does the final projections. But, when we are making maps, we are not just peeling an orange. Our goal is to represent locations on Earth as precisely as possible, in 2D form based on 3D form of Earth. There are various kinds of projection methods used to preserve the originality as much as possible. The following figure describes three major peeling (projection) methods for Earth. Out of the following, C is most common one and the reason why regions/countries closer to the equator appear smaller than regions closer to the poles, which leads to the misrepresented reality of sizes.
So, one could say that projected coordinate system is a flat representation of Earth’s spherical surface. Latitude and longitudes are converted to X, Y coordinates of two dimensions. X coordinate is the eastward direction and Y coordinate is the northward direction of the point of interest. This makes the projected coordinate system more suited to area and distance measurements. The geographical coordinates are converted using mathematical formulas (called map projections) as discussed above. Based on the projection method used, the spatial properties will appear distorted.
You can see an example of this on The True Size, which allows us to see the extent of distortion experienced due to Mercator projection.
Latitude and Longitude:
The imaginary parallel lines running from east to west direction on Earth surface are known as latitudes. The longest latitude is the equator, with their length decreasing as they get closer to the pole. The line running perpendicular to the latitudes, from pole to pole in north-south direction on earth surface are called the longitudes. They are also called the Meridians, with the longitude passing from Greenwich, England named the Prime Meridian. They are equal in length and are farthest at the equator with meeting at poles. The imaginary mesh formed on the Earth surface after the intersection of latitudes and longitudes is called a geographic grid, used to identify location on earth.
GPS:
The Global Positioning System (GPS), along with its equivalents (Beidou in China, Galileo in Europe, and GLONASS in Russia), has transformed the method of measuring position, enabling individuals to know their location almost precisely anywhere on the Earth's surface. The GPS is a complex system comprising of 24 satellites (and a few standbys), each of which travels in a separate orbit around the Earth every 12 hours at a height of 20,200 km while emitting radio pulses at precisely timed intervals. Using the satellite’ exact position and speed of light, a receiver can know the exact location by performing some calculations using the signals. Accuracy of positioning in three dimensions (latitude, longitude, and elevation) depends on the number of satellites above the horizon and their positions. A minimum of four satellites are required to be positioned above the horizon, whereas if elevation is not needed, then only three satellites need to be above the horizon. There are numerous variations of GPS, each with a unique level of accuracy.
Degrees and Decimal Degrees Conversion
Geographic coordinates can be expressed either in degrees–minutes–seconds (DMS) or in decimal degrees (DD). The conversion from DMS to DD follows the relationship: DD=degrees+minutes/60+seconds/3600
Conversely, converting from DD to DMS involves separating the integer part as degrees, multiplying the fractional part by 60 to obtain minutes, and then multiplying the remaining fractional part by 60 to get seconds. Decimal degrees are advantageous for computational applications, whereas DMS notation remains common in navigation and traditional mapping.
Normativity:
Coordinate Reference Systems (CRSs) in Geographic Information Systems (GIS) are deeply embedded with normative assumptions that shape how spatial data is perceived, represented, and applied. Normativity in this context refers to the values, priorities, and judgments that underlie CRS-related choices—from selecting between geographic and projected systems to deciding on specific datums or projection parameters. At a foundational level, the choice of map projection within a CRS involves trade-offs that cannot be avoided: preserving area versus shape, direction versus distance. These trade-offs are not purely technical—they reflect the intended purpose of the data and often privilege certain geographic perspectives over others. For instance, an equal-area projection may be chosen for environmental conservation planning, whereas a conformal projection might be prioritized for navigation, each decision emphasizing different spatial truths while minimizing others. Normativity also emerges in how multiple CRSs are reconciled when integrating datasets. Selecting transformation parameters between CRSs, deciding on tolerances for positional accuracy, or choosing to align with a global versus a local datum all embed assumptions about what constitutes “acceptable” accuracy and which spatial relationships are prioritized. These choices influence how patterns appear in maps, how boundaries align, and which regions receive representational fidelity.
Many widely used CRSs reflect a historically Eurocentric worldview. For example, the Greenwich meridian as the global zero-longitude line and the widespread adoption of projections such as Mercator or Web Mercator center Europe both visually and conceptually. This centrality is not neutral—it privileges certain regions as spatial reference points and subtly frames global geography through a European lens. Such choices shape perceptions of scale, distance, and importance, influencing how space is conceptualized in mapping, education, and policy. Recognizing these origins and their implications is essential for understanding the embedded biases in global spatial frameworks.
As CRSs serve as the backbone of spatial data interoperability, their normative dimensions carry weight in decision-making contexts such as policy development, land administration, and environmental governance. Scholars argue that explicitly acknowledging the values and assumptions embedded in CRS choices fosters transparency and more equitable spatial practices. By critically engaging with the normative aspects of CRS selection and transformation, practitioners can better anticipate the socio-political consequences of their maps and ensure that technical precision is balanced with responsible, context-sensitive representation.
Tutorial on Reprojection:
A common problem is that the data you’d like to use is available only in the wrong projection. The two most used projections are WSG-84 and UTM. For this tutorial, let’s suppose the map you are creating uses the WSG-84 projection while some data you’d like to add as a layer is only available in UTM. WSG-84 displays spatial data in degrees of longitude and latitude. UTM divides the globe in narrow compartments and assigns coordinates within these compartments. The relevant section for our example is ‘UTM 32N’ because this is where Lüneburg is located.
The file used for this tutorial is ‘5_GIS Tutorial_Lueneburg_Special Places_WSG84_EPSG 4326.csv’ which is the location of various places in Lüneburg, Germany.
Add the data via Add Delimited Textlayer, as explained in previous tutorials. Now right click on the layer you just added and select Export → Save Features As.
You will now create a new layer which displays your data in the right projection. You can also create a new csv file, if you wish, but for now, we just create a new shapefile layer.
- Under Format, select ESRI Shapefile
- Under File Name, give your new file a name. I’ll use “lueneburg_special places_UTM 32N”. Specify were to save it via “...”.
- The important step comes now: Under CRS, click on the globe symbol. A new window appears and you can search for “UTM 32N”. Under Predefined Coordinate Reference Systems, please chose “WSG 84 / UTM zone 32N”. The associated Authority ID is EPSG:32632. Click OK
You receive a new layer which is in the UTM 32N projection.
Good job!