GIS data

The field of mapping and GIS has its own language. Below we will go over some of the key concepts and words you might come across when working with map data, especially in a fire management context.

As mentioned briefly before, map data identifies features and positions on the Earth. Within the field of mapping/GIS there are two data types used to represent these features; vector and raster data. The main difference between these two data types is that vector data is presented as a point, line or area, whereas raster data is represented as a grid of pixels as shown above.

In the context of fire management much of our field data and map data will be of a vector type, whilst satellite imagery which is used for mapping burnt areas area in a raster form. Understanding the difference between these types of data is useful for understanding what you can do with them.

1. Vector Data

Vector data maps things on the ground as points (distinct locations), lines and areas also known as polygons. Polygons are line boundaries around the perimeter of a feature. Shown above is an example of a picture of features in the real world, and how they might be displayed as 'vector' map data, or lines and polygons.

In fire management points might be hotspots (active fire locations), bore points or sacred sites; lines might be incendiary tracks, roads or rivers; Polygons might be burnt areas, paddock boundaries or vegetation.

Central to the use of vector data is the fact that each point, line or polygon has a table of information linked to it describing things about it. In the example above there are:

- Hotspot points, each point is linked to a table with information about what satellite it came from and when the point was detected.

- A burnt area polygon describing the area burnt, when it was mapped and an estimate of how hot (severe) the fire was.

- A tenure polygon with information about a piece of land.

- An incendiary track or line recorded from a helicopter with information about when the flight was conducted and who was dropping the incendiaries.

Accessing such attribute information can be very useful for planning your field operation. It can also be important to collect attributes when recording locations in the field so others can see the field information collected about a place. CyberTracker™ is a useful tool for collecting this attribute data in the field.

Attribute data can also be accessed via website maps such as NAFI. In the example above from NAFI the hotspots contain attribute information associated with each point (hotspot) about the date and time it was recorded, source (where the data came from) and which satellite recorded the hotspot.

Just as there are different types of vector data (points, lines, polygons), so too can vector data be stored in a number of different file formats:

- GPX (GPS Exchange format) this is a file type associated with GPS', used for collecting data in the field. It is a format allowing GPS data (waypoints, routes and tracks) to be opened by different GPS devices and software. This allows for easy sharing of data without the need for file format conversion. We will go over GPS and field data collection later in the course.

- KML (Keyhole Markup Language) this is the file type used to display geographic/location data in web-based mapping applications such as Google Earth.

- SHP (shapefile) this is one of the most common vector file types, and can be opened in many GIS programs.

2. Raster Data

Raster data represents the earth's surface as a grid of cells (also called pixels). Each pixel is the same size, and each one represents a location. Each pixel has a number value that represents some characteristic of that region. The satellite imagery from which fires are mapped is one example of raster data. With satellite imagery, each pixel has a number value representing the amount of reflected sunlight captured by the satellite sensor.

The image below shows a region of the Arnhem Land escarpment. When you zoom in we can see the actual pixels making up that image. This is very similar to the way a digital camera or your phone stores photos.

It is common to display vector and raster data together. For example you may have a GPS point you gathered in the field (Vector) and you display them over satellite imagery on Google earth (Raster). Each data type can support our planning work in different ways.

3. Coordinate Systems

A coordinate system is a system of recording a location on the earth's surface - every location can be identified by a unique set of numbers. The two most commonly used coordinate systems you will come across are the Geographic coordinate system that uses latitude and longitude to mark points on the earth, and the Universal Transverse Mercator (UTM) system that uses eastings and Northings.

The Geographic coordinate system draws straight lines from the top to the bottom of the earth; these are called lines of latitude. These lines are measured in the number degrees they are from the equator. Lines around the earth are called lines of longitude. These lines are measured in the number of degrees they are from an imaginary line called the prime meridian. The image above shows this system.

The UTM system divides the world into grid zones (above), and position is indicated by an easting and northing position within one of these grid cells.

Each of these systems has advantages and disadvantages. The UTM coordinate system relies on the metre unit of measure, which incorporates the simplicity of the decimal system and its easy-to-comprehend units of ten. On the other hand, the LAT/LON coordinate system relies on the degree, minute, and second unit of measure, which incorporates the angular system and its cumbersome units of 60. However the UTM system is cumbersome when working across grid zones. In northern Australia which covers a large area of country (and crosses many grid zones), we generally work in a geographic (latitude and longitude) system. However, when working across small landscapes, it can be useful to work in UTM system.