Well, here’s another in my subscriber-only series of posts. Please feel free to share it with your Twitter or Facebook friends. They’ll get the most benefit from it if they download the FREE eBook though. Unless they do that they wont find out about other new posts in this series.
In this video I talk about where you get GIS data from. I’m not talking about the “canned” data you download from some site on the web, but I start to introduce the idea that some GIS projects require mapping specifically for that project, maps that you have to create yourself.
In case some of you missed it in a previous video I briefly introduce the idea of points lines and polygons again and then I quickly move on to talking about some data sources…notably air photos, paper maps and satellite imagery. Each of these three data sources have strengths and weaknesses and I introduce you to some of these. Here’s some of the stuff I talk about…
Air photos: The photos from google and in many corporate GISs are great because they’ve been corrected for photographic distortion. But when you’re doing your own GIS project you sometimes have to use air photos that haven’t been corrected, or very small scale photos (eg. taken from very high up). I’ll go into this in more detail in a later video.
Paper maps: These are a great source of information for your custom projects, but these too come with issues. You need to be sure that the scale of the paper map you intend inputting to your GIS matches the scale of your GIS project. So, if you’re doing a very local project, digitizing roads from a national road atlas will be of little use. Not only will local streets be absent but at very small scales (say 1:1 million), the width of the line on the map may equate to kilometres on the ground. You also need to be sure that you understand the purpose of the paper map you’re using. For example, a geology map might help with a soil map interpretation but it is not a soil map, and a poverty map might help with a crime map interpretation but it is not a crime map.
Satellite imagery: For regional scale projects you just can’t go past satellite imagery. NASA georectifies many of its satellite images these days. By georectify I mean that NASA corrects for photographic distortion and then places the image into a coordinate system. The imagery I’m most familiar with is LandSat imagery. The problem with LandSat imagery is that every pixel equates to 30 metres on the ground so it lacks spatial resolution. However what it lacks in spatial resolution it more than makes up for with its spectral resolution. The LandSat satellite can detect seven bands of spectral data, much of which could not normally be detected by the human eye. the big thing you can detect is moisture which can be used as a surrogate for plant stress. Using remote sensing software you can also “train” an image to detect different types of land cover such as built up areas, roads, crop varieties, forest types, etc. Its very powerful stuff!
Next video: At this stage I’m hoping to talk about map digitizing techniques. If you want me to talk about something different be sure to let me know.
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PLEASE GIVE ME FEEDBACK: This is a young site and I’d like to make it relevant to people wanting to learn GIS. My thoughts are that I’ll start with a few examples of GIS applications, mostly from the eBook before going into detail about GIS theory. I’d really like your feedback about what you want. For example is this video too long for you & would you prefer it to be split up? Or do you just want to know about the GIS techo stuff? Please write you comments below. I make use of the Facebook commenting system – it is a way to ensure less spam on the site.