An Introduction to GIS
Overview
Just as we use a word processor to write documents and deal with words on a computer, we can use a GIS application to deal with spatial information on a computer. GIS stands for ‘Geographical Information System’.
A GIS consists of:
Data –– the geographical information that you will view and analyse using computer hardware and software.
Hardware –– computers used for storing data, displaying graphics and processing data.
Software –– computer programs that run on the computer hardware and allow you to work with digital data. A software program that forms part of the GIS is called a GIS Application.
Some people also include:
People
Methods
Many people consider GIS to just be the software, but it is much more than this. It refers to all aspects of managing and using digital geographic data.
What can you do with GIS?
Identify change
Understand trends
Set priorities
Monitor change
Perform forecasting
Manage and respond to events
Capture, manage and query data
Perform spatial analysis
Present results
It is difficult to briefly explain what you can do with GIS. There are such a variety of different applications. Can you think of a few specific examples that you might want to do with GIS, either in your area of work or further afield?
What is GIS software / a GIS application?
GIS Applications are normally programs with a graphical user interface that can be manipulated using the mouse and keyboard
The application provides menus near to the top of the window (File, Edit etc.) which, when clicked using the mouse, show a panel of actions
These actions provide a way for you to tell the GIS Application what you want to do. For example you may use the menus to tell the GIS Application to add a new layer to the display output
2 examples are shown below:
QGIS, the leading free and open source GIS software
ArcGIS Pro, the latest desktop GIS application from Esri, the leading commercial GIS company.
GIS Data
The data we use in a GIS has a spatial aspect to it, ie. it is tied to a specific location on the earths surface. It can also reference non-spatial data and information, which can be tagged to this location. This a powerful feature of GIS, as it allows us to bring disparate sources of data and information in to a common map framework
GIS data is normally referenced using a coordinate (x, y, z). But it can also be referenced using other reference systems, such as a postcode, or what3words.
The below image is an example of information regarding Sky-Blu camp. The latitude and longitude identify its location in space and the supplementary information gives more context and allows for analysis.
Examples of data where the spatial (geospatial, geographic, georeferenced) data is essential:
We will go through more information about specific types of GIS data in the next section
Coordinate Reference Systems, Datums and Map Projections
Coordinate reference systems
The way we know the location of data is by referencing it against a coordinate reference system (CRS), which can be defined by a projection and/or datum.
For example, a 2D point location will have an x and y coordinate, but simply giving numbers for these coordinates won't tell us anything about where it is. We also need to know what coordinate reference system it is measured against.
When we record the coordinates of spatial data, we should always report the coordinate reference system along with it. Similarly, if we are given coordinate data, we need to determine what coordinate reference system it is in.
A coordinate reference system is defined by a datum and projection.
Map Projections
Map projections try to portray the surface of the earth on a flat piece of paper. In other words, they try to transform the earth from its spherical shape (3D) to a planar shape (2D).
Maps are designed to not only represent features, but also their shape and spatial arrangement. Each map projection has advantages and disadvantages. The best projection for a map depends on the scale of the map, and on the purposes for which it will be used. For example, a projection may have unacceptable distortions if used to map an entire continent, but may be an excellent choice for a large-scale (detailed) map of a country.
The process of creating map projections is best illustrated by positioning a light source inside a transparent globe on which opaque earth features are placed. Then project the feature outlines onto a two-dimensional flat piece of paper. Different ways of projecting can be produced by surrounding the globe in a cylindrical fashion, as a cone, or even as a flat surface.
A projection will generally aim to retain one key aspect of true geometry, at the expense of all others. For example:
Equal area projections will retain the measure of area to conform across the whole projection
Equal distant projections will retain the measure of distance
Conformal projections will retain the true shape of the feature on the earth
You also get projections that compromise all three characteristics, within some acceptable limit. The Winkel Tripel projection and the Robinson projection are examples of these, and these are often used for producing and visualising world maps.
Datums
The earth is shaped like a flattened sphere, called a spheroid or ellipsoid. While a spheroid approximates the shape of the earth, a datum defines the position of the spheroid relative to the centre of the earth. A datum provides a frame of reference for measuring locations on the surface of the earth. It defines the origin and orientation of latitude and longitude lines.
For Antarctica, the datum most commonly used is WGS84. It is also the datum adopted by the GPS system, and provides a convenient global datum solution for a lot of purposes.
Because not all latitude longitude datum systems use the same underlying ellipsoid model, a single latitude longitude coordinate will not reference the same place on earth, if those coordinates have been measured against different datums. In the UK for example, there is a 120 m datum shift between the lat/long of WGS84, and the lat/long of OSGB 36 in East Anglia. The shift varies across the UK. It is therefore essential to know what datum your lat/long coordinates are in.
Don't worry if you don't fully understand CRSs, projections and datums. They can seem complicated to start with. If you want to do any extra reading, then there are many useful resources including this page by PressBooks.
EPSG codes
EPSG codes provide a standard way to identify a coordinate reference system.
EPSG codes are built into GIS software, such as QGIS and ArcGIS. It is common practise to use EPSG codes to identify coordinate reference systems for datasets.
Codes can identify a datum, or a projection, with associated datum.
EPSG:4326 is the code for WGS84 datum
EPSG:3031 is the code for WGS84 / Antarctic Polar Stereographic projection
Refer to https://spatialreference.org/ to find any coordinate reference system, and it's associated EPSG codes and definitions.
What projection should I use for my project or map?
We get this question a lot. Here are a few of the most common projections that we recommend:
Antarctic Polar Stereographic, EPSG 3031: good general projection for showing large parts or all of Antarctica
If you need to calculate areas, then work in an equal area projection. For Antarctica, this is South Pole Lambert Azimuthal Equal Area, EPSG 102020
Specific UTM zone projections: for smaller regions we often use these. The world is split into these zones. Rothera is 19S and the South Orkney Islands are 23S.
Lambert Conformal Conic projections are useful for medium sized areas. For South Georgia, we use South Georgia Lambert, EPSG 3762
This website called Projection Wizard is very helpful, if unsure.
Information sources:
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