GIS Fundamentals: A Beginner's Guide

What is GIS?

GIS stands for Geographic Information System. It is a technology used to collect, store, analyze, manage and visualize information that is connected to a location on Earth. If you have used Google Maps to find the nearest hospital or the best route, you have used technology that GIS helps power.

The connection to location is what makes GIS different from a normal database or spreadsheet.

Maps are an important part of GIS, but they are one output. The larger use is asking what is happening somewhere, why it is happening there, and what is nearby.

What can GIS answer?

GIS can help answer questions such as:

• Which houses are within 5 km of a hospital?
• Which villages are affected by flooding?
• Where should a new cell tower be installed?
• Which agricultural fields are showing poor crop health?
• Which roads are closest to a proposed market?
• How has a city's built-up area changed over the last 10 years?

The basic components of GIS

A GIS project usually involves two kinds of information.

Spatial data tells us where something is. For example:

• Roads
• Buildings
• Parks

Attribute data tells us more about the spatial data. For example, a road might have these attributes:

• Road name
• Road type
• Length
• Speed limit
• Condition

GIS vs GPS vs remote sensing

GIS, GPS and remote sensing are often mentioned together. They are related but do different jobs.

What is GPS?

GPS stands for Global Positioning System. It is a satellite-based positioning system that determines the location of a receiver on Earth. Your phone uses GPS when you:

• Navigate using Google Maps
• Track a running route
• Share your location
• Book a cab
• Record a field survey location

For example, if you stand outside a building and record its coordinates with your phone, GPS can give you something like 28.6139° N, 77.2090° E.

That tells you where the location is. But GPS does not automatically tell you what is happening at that location.

What is remote sensing?

Remote sensing is the process of collecting information about the Earth's surface without physically touching or directly measuring the object. Satellites and drones are common examples.

A satellite such as Landsat or Sentinel-2 captures electromagnetic energy that is reflected or emitted from the Earth's surface. Different materials interact with that energy differently. For example:

• Healthy vegetation reflects strongly in the near-infrared region.
• Water behaves differently from vegetation.
• Bare soil has its own spectral characteristics.
• Built-up areas have different reflectance patterns.

This allows us to analyze things that may not be obvious from a normal photograph.

Where does GIS come in?

GIS is where these datasets are combined and analyzed. Take a field-monitoring workflow:

Vector vs raster data

Vector and raster are the two fundamental ways GIS represents geographic information.

What is vector data?

Vector data represents geographic features using points, lines and polygons. Think of it as drawing individual objects on a map.

Point. Used for things that can be represented by a single location. Examples:

• Schools
• Hospitals
• Trees
• Weather stations

Line. Used for features that have length. Examples:

• Roads
• Rivers
• Pipelines
• Railway tracks

Polygon. Used for areas. Examples:

• Buildings
• Lakes
• Land parcels
• Administrative boundaries
• Agricultural fields

For example, a city's boundary could be represented as a polygon, while the roads inside it could be represented as lines.

What is raster data?

Raster data represents the world as a grid of cells, or pixels, and every cell contains a value.

Satellite imagery is one of the most common examples. A satellite image is made up of millions of tiny squares. Each square represents a small area on the ground and stores information about that location.

Raster data is commonly used for:

• Satellite imagery
• Elevation models
• Temperature
• Rainfall
• Land-cover maps
• Vegetation indices such as NDVI

What are shapefiles?

If you have worked with GIS for even a short time, you have probably opened a folder containing files like these:

roads.shp
roads.shx
roads.dbf
roads.prj

What is a shapefile?

A shapefile is a popular geospatial vector data format originally developed by Esri. It can store geographic features such as points, lines and polygons.

A shapefile is not one file. It is a collection of files that work together.

The files that make up a shapefile

The .dbf file. If you have a roads layer, the attribute table might look like this:

The geometry tells you where the road is. The DBF tells you what information is associated with that road.

The .prj file. Without knowing the CRS, your data may not line up correctly with other spatial datasets. For example, the .prj file might tell GIS software that your data uses WGS 84 / UTM Zone 43N.

Shapefiles remain common because almost every GIS application supports them, but they have several limitations.

Shapefile limitations

• The .shp and .dbf files are each limited to 2 GB.
• Attribute column names are limited to 10 characters, so longer names get truncated.
• A single shapefile cannot mix points, lines and polygons.
• A single layer is spread across several files, which makes it easy to lose or break one by accident.

This is why modern workflows often use formats such as:

• GeoPackage (convenient, because it keeps everything in a single file)
• GeoJSON
• File Geodatabase
• PostGIS

What is a coordinate reference system (CRS)?

Suppose you have two maps of Delhi and want to put one on top of the other. If they use different coordinate reference systems, your roads might appear several kilometres away from where they should be.

What is a CRS?

A Coordinate Reference System (CRS) defines how locations on the Earth's surface are described using coordinates, such as 28.6139, 77.2090.

But those numbers do not mean much by themselves. We also need to know which coordinate system they belong to.

Why do we need a CRS?

The Earth is not flat, but the maps displayed on your computer are. So we need mathematical methods to represent locations on the curved Earth on a flat surface. Different CRSs use different approaches for doing this.

Geographic CRS. A geographic CRS represents locations using latitude and longitude. For example: Latitude 28.6139°, Longitude 77.2090°. These are angular measurements. Geographic coordinates are useful for identifying locations globally.

Projected CRS. A projected CRS converts geographic locations into a flat coordinate system, usually using units such as metres. For example, the same location in UTM zone 43N is Easting 715,980 m, Northing 3,167,205 m. This can be much more convenient for measurements and spatial analysis.

Why a CRS matters in everyday GIS

Suppose you have:

• A road layer
• A satellite image
• A village boundary
• GPS points

If they use different CRSs, they may not line up correctly. GIS software can often transform one dataset into another CRS, but you need to understand what you are transforming and why.

Assigning vs reprojecting a CRS

These two actions sound similar, but they do very different things.

In QGIS, you can do this as follows:

• Check or assign a CRS: open the layer properties and look under the Source tab.
• Reproject a layer: use Layer → Export → Save Features As… and pick the new CRS, or run the Reproject layer tool from the Processing Toolbox.
• See the project CRS: look at the bottom-right corner of the QGIS window.

Geographic vs projected CRS

One of the first things that confuses people learning GIS is the difference between a geographic CRS and a projected CRS.

Geographic CRS

A geographic coordinate reference system represents locations using angular coordinates: latitude and longitude. For example, 28.6139° N, 77.2090° E. A very common geographic CRS is WGS 84 (EPSG:4326).

Geographic coordinates are useful for identifying locations globally, but latitude and longitude are angles, not metres. A difference of 0.001 degrees does not represent the same physical distance everywhere.

Projected CRS

A projected CRS takes the curved Earth and represents it on a flat coordinate system. Coordinates are commonly measured in metres, for example Easting 715,980 m and Northing 3,167,205 m. This makes projected CRSs very useful for spatial analysis.

For example, if you want to create a 5 km buffer around a set of roads, a CRS that uses metres is much more convenient than working directly with latitude and longitude.

Why can't we use one CRS for everything?

Because every projection involves some form of distortion. You cannot flatten the Earth's curved surface without changing something. Depending on the projection, you may distort:

• Distance
• Area
• Shape
• Direction

Different CRSs are therefore designed for different purposes.

What is an EPSG code?

You have probably seen numbers such as EPSG:4326, EPSG:3857 and EPSG:32643. These are called EPSG codes.

What EPSG stands for

EPSG originally referred to the European Petroleum Survey Group. Today, the EPSG Geodetic Parameter Dataset is maintained by the International Association of Oil & Gas Producers (IOGP). The dataset contains standardized information about coordinate reference systems and related geodetic parameters.

An EPSG code provides a standardized way of identifying a particular CRS.

Some common codes

Choosing a CRS

An EPSG code identifies a CRS but does not decide which one you should use. That depends on:

• Your geographic location
• Whether you need distance or area measurements
• The type of analysis
• Map scale
• Required accuracy

WGS84 and EPSG:4326

If you work with GIS, GPS or mapping, you will encounter WGS84 constantly. It is one of the most widely used geographic reference systems in the world.

What is a datum?

Before looking at WGS84, it helps to know what a datum is. A datum defines the size and shape of the model of the Earth (an ellipsoid) and how that model is positioned relative to the real Earth. Coordinates are measured from this reference.

Different datums can give slightly different coordinates for the very same place, sometimes by several metres or more. That is why a coordinate only makes sense when you also know its datum.

What is WGS84?

WGS 84 stands for World Geodetic System 1984. It is a global geodetic reference system used to define positions on Earth. It is closely associated with GPS and is widely used by mapping systems and geographic datasets.

When someone gives you coordinates such as 28.6139, 77.2090, there is a good chance they are latitude and longitude in WGS84. You should always check the metadata rather than assume.

What is EPSG:4326?

EPSG:4326 identifies the WGS 84 geographic CRS. It uses latitude, longitude and degrees. For example: Latitude 28.6139°, Longitude 77.2090°.

Why is WGS84 so popular?

Because it is global and widely supported. You will find WGS84 used in:

• GPS
• GeoJSON
• Web mapping
• Mobile applications
• Global datasets
• Many satellite and remote-sensing workflows

Is EPSG:4326 good for measuring distance?

Not for accurate work. EPSG:4326 uses degrees, not metres. To calculate accurate local distances, or to create a buffer such as "10 km around this road", you will usually want an appropriate projected CRS rather than working directly in geographic coordinates.

UTM explained

Delhi at 28.6139, 77.2090 becomes 715980, 3167205 in UTM zone 43N. That is why some GIS datasets have coordinates that look nothing like latitude and longitude.

What is UTM?

UTM stands for Universal Transverse Mercator. It is a projected coordinate system that divides the world into 60 longitudinal zones. Each zone is 6 degrees of longitude wide, and the zones are numbered from 1 to 60, from west to east.

Why divide the world into zones?

Projecting the entire Earth onto one flat map would introduce significant distortion. UTM instead divides the Earth into smaller zones, each with its own projection, which keeps distortion limited within a zone.

UTM coordinates

UTM uses easting and northing rather than latitude and longitude, and the coordinates are measured in metres. Delhi, for example, is Easting 715,980 m, Northing 3,167,205 m. That makes UTM particularly useful for many local and regional GIS calculations.

Northern and Southern Hemispheres

UTM zones are also divided by hemisphere. For example, UTM Zone 43N means Zone 43 in the Northern Hemisphere. The corresponding Southern Hemisphere zone would be written 43S.

How do you find your UTM zone?

A simplified formula for the zone number is:

Zone = floor((Longitude + 180) / 6) + 1

For example, Delhi lies at around 77° E, which places it in UTM Zone 43N. For WGS84, that CRS is EPSG:32643.

When should you use UTM?

UTM can be a good choice when:

• Your study area is reasonably localized
• You need coordinates in metres
• You are performing distance or area-based analysis
• Your data falls primarily within one UTM zone

For example, if you are analyzing agricultural fields in one region, a suitable UTM CRS can make measurements and spatial operations much easier.

EPSG:4326 vs EPSG:3857

Both EPSG:4326 and EPSG:3857 are based on WGS 84, but they are used for very different purposes.

EPSG:4326

EPSG:4326 is the WGS 84 geographic CRS. It is useful when you want to describe a location using geographic coordinates, and you will meet it frequently in GPS data, GeoJSON, global datasets, location APIs and geographic databases.

EPSG:3857

EPSG:3857 is commonly called WGS 84 / Pseudo-Mercator. It uses projected X and Y coordinates, with units that are approximately metres. It is widely used by web mapping platforms because the projection works conveniently with tiled web maps. You will encounter it in online maps, web map tiles, interactive mapping and mapping libraries.

Why does web mapping use EPSG:3857?

Web maps need a projection that works efficiently for displaying the world on a computer screen, and Pseudo-Mercator became widely adopted for this purpose. However, it comes with significant distortion, particularly as you move toward the poles.

So which one should you use?

• EPSG:4326: "Where is this location on Earth?"
• EPSG:3857: "How do I conveniently display the world in a web map?"

Georeferencing

Sometimes you have a map as a scanned image or a photo, and it has no location information at all. Georeferencing is the process of giving that image real-world coordinates, so it lines up with your other GIS layers.

How georeferencing works

1. Scan or load the image you want to place on the Earth.
2. Choose control points. These are features you can identify both on the image and on a reference map, such as road junctions or building corners.
3. Enter the real coordinates of each control point, or click the matching spot on the reference layer.
4. Pick the target CRS for the result.
5. Choose a transformation method. This decides how the image is stretched and rotated to fit the points.
6. Save the output as a new georeferenced raster, such as a GeoTIFF.

Popular GIS tools

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