GIS: DATA FROM SPACE BECOMES A MAP

Map of temperatures of Earth

What are Geographic Information Systems (GIS)?

In computer speak, information systems are a way of dealing and working with information digitally (using computers). GIS are special computer information systems, which we can apply to any data as long as we know where on the earth it is found. To do this we need to know the geographical coordinates (longitudes and latitudes) of the target area. We can use GIS to take the information that is imbedded in the satellite image and make a map with many layers built into it. This helps us to extract information about the target area so we can see aspects of the target that may have been hidden from us by just using the naked eye.

GIS tries to model target areas on the earth. The process involved in making these models includes collecting and storing data made available from remote sensing (read our article about remote sensing here). The data is stored in repositories called geodatabases. This data is analysed in such a way that each piece of information the data holds can be separated into its own unique layer. By adding or removing various layers, GIS makes it possible for the target area to be visualised in different ways on a map.

GIS Layers

For example, if you want to know about the area where you live, you can take a satellite image and add a map of the rivers, the roads, the schools, the hospitals, even the best malls. GIS puts these maps in layers one above the other, so you can see all the information at once. Or, you can remove information that doesn’t interest you, for instance the school map. Then you can add the maps that you want, such as where all the cool malls are. So, with GIS you can analyse data to help you answer important questions. You can then plan and make decisions about the best entertainment and fast food joints nearby in the real world.

The Data

Attribute data
Attribute data is usually a spread sheet with longitudes and latitudes included

Geodatabases are like files or libraries where the data of every target area on Earth, are stored. There are important parts to this data. All these components must be present for GIS to work and create maps, which are the windows for exploring the data. One of these is called Geospatial data. This data provides information about the geographical co ordinates of the target area, the type of map projection and the resolution of the image. In other words, it tells you where in the world your data is located.  Attribute data is usually in table form, such as an excel spread sheet that has columns for latitude and longitude. It tells you what the data is about, for example rainfall or land use types. Attribute data includes information about whether the model of the data is represented as a vector, which would be read by the computer as a shapefile, or a raster, which would be read as an image like a .tif file, or a .geotif file, or .jpeg.

The Models

GIS models represent real world things either as vector data models, or raster data models.

Vector data models

Vector data: Polygon showing boundary of Zinkwazi Beach, South Africa

Vector data models represent real world features that can be outlined in shape form. The spatial geometry of vectors is made up of one vertex or interconnected vertices. A vertex describes a position in space using X and Y coordinates, and sometimes even a Z coordinate. This shows the height of the feature above the earths’ surface. If the spatial geometry has a single vertex, it is a point feature and X: Y coordinates represent a location such as a school or a church. If the geometry has two or more vertices; it is called a polyline or line feature such as a river or a road. If there are four or more vertices and the last vertex is in the same place as the first one; an enclosed polygon feature is formed like the boundary of a country or the perimeter of a lake.

Some features can’t be shown easily using vectors, such as grasslands. The vector data model can make a polygon around the whole grassland but lots of information is lost, like the different colours of grass species and plant densities. This is because when you give a feature attribute values, they apply to the whole feature in a vector data model, and they can’t be divided into smaller units that may give more information.

Raster data models

Raster Data: Orthophoto map showing point and polygon vector data of Zinkwazi Beach, South Africa
Raster base map overlaid with point and polygon vectors

In order to get this extra information, you can use raster data models. Raster data is represented in a matrix of pixels. Each pixel has an attribute (e.g. elevation, rainfall, land use, etc). Satellite images and aerial photos are in this format (e.g. Google Earth). So, people often use a raster layer behind a vector layer, because it makes the vector layers stand out so that they are easier to interpret.

The GIS modelis therefore the plan that you make about what question you need to have answered about a place of Earth and how you can use the image data and place it into a GIS program in order to make a map of different layers  that can maybe make certain facts about the area stand out in such a way that it is easier to answer your question.    

Vector topography map overlaid on a raster base map of Zinkwazi Beach, South Africa
Vector topography map overlaid on a raster base map of Zinkwazi Beach

The Maps

Maps must be able to tell a story using special information, that anybody can understand clearly. They try to show the surface of the Earth, or a portion of it, on a flat piece of paper or a computer screen. They need a map body, a title, a legend, a north arrow, a scale bar, acknowledgements and a map border.

Map projections

The Earth is not flat, no matter what the Flat Earth Society would have you believe. But it is not round either. It is geoid shaped; irregular, non-mathematical, and as we all know, often chaotic. We have to use a sphere to represent the Earth because it is closest shape to a geoid that we can divide up mathematically. A map projection provides various ways to project maps of the Earth’s surface onto a 2-dimensional cartesian coordinate plane. That’s fancy geometrical language for a piece of paper! There are three types of map projections; cylindrical (e.g. Mercator projection), conical and planar (e.g. Azimuthal projection) projections, with different variations of each. For example, UTM (Universal Transverse Mercator) is an internationally used projection system, which is a variation of a cylindrical projection.

Werner map projection
There are many different ways to represent the spherical Earth on a chart. The Werner map projection of the Earth, Attributed to Strebe [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]

All of these projections work well in some parts of the Earth, but are grossly exaggerated in other parts. That’s because everyone knows that you can’t squash a sphere onto a piece of paper without getting some distortions somewhere! The problem is that sometimes one of your GIS layers may be projected using a different map projection to another, so when you open the GIS, they will not be located near each other. At the beginning of a project, you can define the projection you want all your layers to be in, and they will then be placed one on top of the other. This is called an “on the fly” projection.

How do we use GIS in our day-to-day life?

Our cell phones and laptops have Global Positioning Systems (GPS) devices built into them. They work out where exactly we are geographically placed on the earth, and GIS adds all the information we need to know about where we are in the form of maps. So, every time we use google maps or Waze, to find our way to a destination, we are using GIS. Also, if we look at weather maps to predict what to wear in the morning when we wake up, or how far we’ve jogged and how steep the gradient was. Even your ID card has a spatial component that relies on GIS!

Cell phone showing map
Recognise this? We use GIS everyday

Other uses of GIS

Ecologists use GIS to track endangered animals which have been fitted with radio transmitting collars. The positions of the animals can be overlaid on maps of vegetation, soil type, rainfall or even climate change maps, so that patterns of these precious animals’ behaviour can be studies.

Architects use GIS to build 3D maps for urban planning. They can work out which direction a building should face taking into account the amount of shadow that will be cast on the building throughout the year; where the sun will shine through to give the best light and even which direction the prevailing winds come from.

Similarly, farmers can use maps showing the aspect of their lands to work out the best position to plant crops throughout the year, and even work out which side of the field the plants will be thirstier and therefore will need more irrigation.

Town planners use GIS to decide how far apart schools, hospitals and police stations need to be placed, and emergency workers use GIS to determine the quickest route from an emergency location to a hospital.

From space to your front door

 It seems we’ve travelled a long road. We started with light captured by a satellite, which gave us data that needed to be assembled into a spreadsheet, each column with different variables including our X and Y coordinates. Then we applied a model that would best help us answer a question in the form of a map. Our GIS software converted data from the spherical world into the two-dimensional world of a map. Finally, we have a map on paper or electronic format that can help us get to the movies on time.

REFERENCES

https://www.nationalgeographic.org/encyclopedia/geographic-information-system-gis/

https://helpwiki.evergreen.edu/wiki/index.php/ArcGIS_10:_What_is_a_Geodatabase%3F

https://study.com/academy/lesson/geospatial-data-definition-example.html

http://geokov.com/education/map-projection.aspx

https://www.quora.com/What-is-the-difference-between-Geospatial-data-and-attribute-data-in-GIS

http://support.pitneybowes.com/SearchArticles/VFP05_KnowledgeWithSidebarHowTo?id=kA180000000Cu9DCAS&popup=false;&lang=en_US

https://techterms.com/definition/gps

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