5. Raster Data

Duration: 10 minutes and 22 seconds

Country: South Africa

Language: English

License: CC - Attribution Non-commercial No Derivatives

Genre: Instructional

Producer: Chief Directorate: Spatial Planning & Information, Department of Land Affairs, Eastern Cape, South Africa

Views: 171 (18 embedded)

Posted by: giacomo on Mar 30, 2010

Discover the different kinds of raster data and how they can be used in a GIS. Raster data are arranged in a matrix of cells. Each cell in the matrix has a value. When viewed as a whole the raster looks like an image. In this worksheet we take a closer look at raster data.

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Hi, my name is Marcelle.
Welcome to the 5th topic in our Gentle introduction to GIS series.
In previous topics we looked at vector data.
We saw that vectors have a geometry, ...
... either: point, polyline or polygon, which are made up of vertices.
In this topic we will take a closer look at raster data.
Raster data are different to vector data.
They have no geometry.
Instead, raster datasets are made or a regular grid of cells or pixels.
Each pixel can be referred to by its column and row position.
The raster grid is georeferenced.
Being georeferenced means that the GIS knows ...
... where to display the raster relative to the earths surface.
In this tutorial, we will load some different kinds of raster data ...
... and see how it can be used in the gis environment.
Let's take a closer look!
One common form of raster data are scanned toposheets.
If you have worked with toposheets in the classroom, these should look familiar to you!
The data is created from paper mapsheets and georeferenced.
It's like having a paper map in your computer!
Let's load a toposheet raster!
Click on the 'Add raster layer' icon in the toolbar.
Navigate to your local data directory under:
... C: ...
... program files ...
... Quantum GIS ...
... Gisdata ...
... Local.
Make sure that you have the 'Files of type' option set to geotiff.
Now select the toposheet_clip file.
And then press OK.
You will see an image appears in the map view.
Notice how the image looks the same as a paper mapsheet.
Also note that compared to vector layers which normally contain one feature type only, ...
... such as rivers, our raster layer represents many feature types!
Another difference between raster and vector data is that raster layers do not have attribute tables.
If I use the 'Identify features' tool to click on the raster, all I get back is a single number.
The number represents the value of the pixel where I clicked.
Let's add some tourism vector data that I created as part of the topic on digitising.
Click on 'Add vector layer' and add the tourism_lines layer from your local folder.
You will see that the vector layer lines up exactly with the raster layer.
This is because the raster layer is georeferenced.
Another thing you can see is that as we zoom in (using the zoom in tool here), ...
... the raster layer starts to become less clear.
In fact if we zoom in enough we can start to see that the raster ...
... is indeed just a grid made up of coloured pixels!
Notice though that no matter how much we zoom in, ...
... the vector layers always keep their nice appearance.
Let's look at another example of a raster layer.
Click on the 'Add raster layer' icon in the toolbar.
Now choose: nasa_jpl_clip.tif
A new layer is added to the map view.
Drag the layer below the vector layers.
The last raster layer we opened looked like a toposheet.
This layer looks more like a photograph.
It is an image by a satellite orbiting the earth.
Satellite images can be useful to use as backdrops for digitising vector data.
A satellite image is also a good way to understand the landscape.
Many features cannot be represented well in vector layers.
For example all of these different vegetated areas would be hard to capture as vector data, ...
... but they are easy for your eye to detect in a raster image.
Images that show on the screen in colour achieve this by using three bands ...
... Red, Green and Blue
If you open the raster layer properties by double clicking on the raster layer in the legend, ...
... you will see the bands listed.
You can change the appearance of the image by changing the band order.
Let's change the order to 321 and click 'apply'.
Some images have more than 3 bands, covering a range of the light spectrum ...
... from infra-red through to ultra-violet.
By using the band mappings you can map those colours that would not normally be visible ...
... to the human eye to the red, green or blue components of the image you see on screen.
You can also change the image to show as grey only by clicking on 'Single band' ...
... in the raster properties and then click OK.
This brings us to the end of our introduction to raster data.
In our next screencast we will look at vector topology.
See you next time!

5. Raster Data

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