Understanding Map Projection in GIS: A Beginner’s Guide
Geographic Information Systems (GIS) find extensive application in mapping, spatial analysis, urban planning, environmental studies, and remote sensing. Map projection is one of the important concepts in GIS, which allows us to depict the Earth’s three-dimensional surface on a two-dimensional map.
Since Earth is a sphere but maps are flat surfaces, any effort to convert geographical locations to flat maps will cause some form of distortion. It is important to understand different kinds of map projections for the proper selection of coordinate systems in GIS and other fields.
What Is a Map Projection in GIS?
The process of mapping locations on the three-dimensional surface of the Earth onto two dimensions involves a mathematical process known as a map projection.
Map projections enable GIS users to create maps and make calculations for distances, areas, and other types of geospatial analysis.
A map projection changes geographic coordinates (latitude and longitude) to projected coordinates (X and Y).
For example, while geographic coordinates give the location of something using angle measurements, projected coordinates will give the same location using a measurement like meters or feet.
However, not all properties of the three-dimensional surface of the Earth can be transferred onto a flat map. Distortion of different properties depends on the type of map projection being used.
Why Are Map Projections Important in GIS?
Projections affect the way that spatial data is presented, analyzed, and measured. An appropriate choice of a projection is crucial to getting GIS outputs that are accurate.
Some of the uses of map projections are as follows:
Analysis of spatial information: Helps measure distances, areas, buffering, and spatial relations.
Cartographic representation: Defines the look of geographic entities on electronic and paper maps.
Remote sensing: Useful for alignment of satellite imagery, aerial photography, and other raster datasets with geographic referencing systems.
Engineering and surveying: Used in coordinate-based measurements and engineering.
Urban and regional planning: Useful for roads, parcels, buildings, and administrative boundaries analysis.
Environmental monitoring: Useful for watershed analysis, land use mapping, and ecosystem assessment.
The inappropriate use of projections can result in incorrect measurements or inaccurate geographic entity representations, especially when performing analysis over a large area.
How Do Map Projections Work?
Map projections employ mathematical formulas that convert geographic coordinates to plane coordinates depending on the type of projection used and its parameters.
The GIS workflow includes the following:
Identification of the geographic coordinate system: The definition of the method used to specify the latitude and longitude and geodetic datum of the source data.
Selection of the map projection: It depends on the size of the area covered and the requirements of analysis.
Transformation of coordinates: Converting geographic coordinates to projected X-Y coordinates.
Spatial alignment validation: To determine whether datasets use compatible coordinate reference systems and are aligned well.
Measurement validation: Distances, areas, and other measures should be validated against projection-induced distortion.
GIS applications such as ArcGIS Pro and QGIS can carry out coordinate transformations and reproject the datasets if the coordinate system information and its transformation parameters are known.
Common Types of Map Projections
Map projections can be categorized based on their geometric properties and the geometric surfaces used to create them.
Cylindrical Projection
Cylindrical projections are map projections where Earth's surface is projected on a cylinder. They can be useful in creating world maps and for navigation purposes.
A good example of such a projection is the Mercator projection, which preserves local angles and shape. But it exaggerates the size of regions in high latitudes.
Conic Projection
Conic projections use a conical geometric surface to represent Earth's surface. They are usually applicable in mapping regions spanning predominantly from east to west at middle latitudes.
Lambert Conformal Conic projections are used in creating aeronautical charts, regional maps, and weather maps since they preserve local angles and shape.
Azimuthal Projection
An azimuthal projection represents Earth's surface through the projection of the plane intersecting or tangent to the Earth's sphere. Properties of distortion in such projections vary based on the specific type of the projection and its center point.
Azimuthal Equidistant projection is characterized by constant scale between all points and the center and equal bearing angles. It can be used for some purposes related to communications, aviation, and other radial distance measurements.
Transverse Mercator Projection
Transverse Mercator projection is created using a cylinder that crosses Earth's meridians, not the equator. It is convenient for mapping relatively narrow areas that stretch from north to south.
UTM (Universal Transverse Mercator) projection system covers most parts of the Earth divided into 60 longitudinal zones, which have their own coordinate systems.
How to Choose the Right Map Projection
The choice of the map projection is determined by the purpose of the map, its location and size, and also by which properties should be maintained.
Here are some considerations:
Location and size of the geographic area: Select a projection intended for that area.
Measurements required: Choose an equal-area projection if there is a need for area comparison, or select a suitable local projected CRS if there is a need for measuring distances and making engineering calculations.
Compatibility: Make sure that all input data have coordinate reference systems.
Distortion of the projection: Consider how the chosen projection affects shapes, areas, distances, directions, and scales.
Software and standards: Consider the organization's mapping standards and coordinate systems available in the GIS tools.
For example, a local engineering project might benefit from using a suitable local projected CRS, and a world thematic map depicting differences in deforestation between different countries might need an equal-area projection.
Map Projections in ArcGIS Pro and QGIS
Modern Geographic Information System applications offer ways of working with coordinate reference systems and performing spatial data transformations.
In ArcGIS Pro, users are able to assign coordinate systems to datasets, set map coordinate systems, and perform projections of spatial data using geoprocessing tools. The Project tool helps transform vector features to another coordinate system, while the Project Raster tool does the same with raster datasets.
In QGIS, users are able to set the project CRS, check the layer coordinate systems, and export datasets to another CRS. On-the-fly reprojection makes it possible to show layers with different supported coordinate reference systems.
It is crucial to draw a clear line between assigning a CRS and reprojecting data. Assigning a CRS means identifying the coordinate system used by existing coordinates. Reprojection is the mathematical transformation of coordinates from one coordinate system to another.
To obtain correct results, users have to double-check the chosen datum transformation, coordinate units, and geographic extent of the projection.
Common Map Projection Mistakes to Avoid
Novices in GIS may come across some pitfalls when using map projections:
Using the wrong projection: Might distort spatial information and result in inappropriate measurements.
Misinterpreting CRS settings and reprojection: Could put datasets in the wrong place.
Mishandling different coordinate units: Could make it difficult to work with meters, feet, and angular coordinates.
Neglecting datum transformation: Might cause problems with positioning of datasets.
Using a world projection to do regional or engineering measurements: Might bring extra distortion to the picture.
Selecting a projection depending on how the map looks: A correctly looking map does not necessarily mean it will be correct for quantitative analysis.
Knowing this helps to avoid some spatial data issues and misleading GIS results.
Map projections are a key component in GIS because they allow transforming the curved Earth into a map in order to work with it. Depending on the purpose, different projections help to preserve different geographic characteristics, which makes the choice of the right projection a very important issue for GIS.
GIS novices will be able to use suitable coordinate systems after learning about different projections and related topics.
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