Join Gordon Luckett for an in-depth discussion in this video Exploring types of raster data in QGIS, part of Learning QGIS (2015).
- Raster data is data that is image based. We can add various types of raster data to QGIS including digital elevation models and orthophotos. Let's look in QGIS to see what kind of raster data it supports. Under Layer, click Add Layer. We can see that there is an Add Raster Layer menu here, but there's also a Add WMS/WMTS, that's Web Mapping Service or Web Mapping Tile Service. Both are streaming raster data from the web service.
Although the original data that is created before the web mapping service sends us data may be vector, it is definitely raster when we receive it, so I consider the WMS and the WTS layers as raster. The Oracle GeoRaster layer is raster data, could be ECW, could be TIFF, stored inside of the Oracle database as a special type. So when you query Oracle, you can get images back from Oracle when you query it. Let's look at the Add Raster Layer.
Here, we're prompted for a file. Some of the files that we can look at are ARC/INFO GRID files or export E00 files, there's ERDAS Compressed Wavelets, ECW, which we might use, JPEG 2000, IMG files, TIFF files, very popular, the GeoPDF as well, can be used, all the grid files. If we scroll down some more, we'll see regular JPEG, MBTiles, which are used for making tiles for the internet.
This is used often by products such as Leaflet, MrSID, you'll see that. If you keep going down to more grid files, PNG and so on. There's many types of files that are supported. Now, the one thing in common with all these files is that they need to be georeferenced. That means they need either a helper file, like a world file for a TIFF file or the coordinates are saved within the file itself. We cannot add a file into our map if we don't know what coordinate system it is or it has not been georeferenced yet.
Luckily, cancel out of here, under the raster, there is a georeferencer tool for those who want to take an image, for example, if I were to lean out of a plane and take a photograph of the ground and I wanted to add that to QGIS, I would need to tell QGIS where that image is. It doesn't automatically have the location and coordinates on the ground. I need to make sure that the images that I add, the raster that I add to my map, have a georeference file or are georeferenced at some point.
Luckily, inside of QGIS there is a georeferencer so that we can add those extra files. Now, understanding the various types of raster that you can use really help you to plan what kind of files you use within your project and whether or not they are georeferenced are critical to your overall project.
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