Duncan Golicher
Both these systems share and visualise Geographical Information and permit specialised analyses
Modern GIS systems use a variation of the “client - server” model used by data bases.
So navigator on a mobile phone is a client application, which receives information from online servers and uses it to help you get from A to B. It can also receive updated information on traffic conditions. The same general approach can be used for monitoring animal movements.
We are used to obtaining information online through a web browser. However much of this information is unstructured, or semi-structured at best. If we “scrape” coordinates from a web site we would be faced with many data processing steps before we could make a usable map. Spatial data bases and map servers deliver spatial information to clients in an immediately usable form.
A data base is a centralised, single repository for data. For example Google itself uses one single data base that serves all queries made. Analysis that changes data stored on the server is available to all users. However once the results from a query are provided to a client they can be analysed and processed locally. Local, desktop GIS processing is not shared with other users.
Operations that involve large amounts of information, particularly information that can be updated in real time, are run on the server. So if, for example, we wanted to calculate the waiting time for pizza delivery to police stations in Manchester we would run a query on the server that looked at the network connecting police stations with pizzerias.
If we were designing a pizza ordering app for hungry policemen we would do this “client side”. The app would receive updated information if a new pizzeria opened up. An app designed for Manchester could also work in Leeds or Bournemouth if it tapped into a national data base.
The GIS professional providing a specialised service to hungry policemen would program an app that itself provided an user friendly point and click interface through a web page that the local constabulary could use. You could of course also imagine a more serious application that sent policemen out to combat crimes as they are reported. However client applications such as ArcGIS or QGIS also allow analyses to be designed using a GUI (Graphical User Interface). This is useful for bespoke analyses that are not
If you only run an analysis once (i.e. you only want to order a single pizza) it can be convenient to run through the steps using a GUI based desktop analytical tool. If you order pizzas every few minutes you would not want to do this.
Most modern desktop GIS software and web mapping applications link to online “base maps”. These are pictures of the earth. An example is Google maps, but there are many others. These are great for looking at the planet, but you can't actually query the information directly. Both raster and vector layers loaded into a GIS can be queried and manipulated locally. Information stored in a spatial data base can also be queried.
A base map is a raster layer (image) consisting of three bands (RGB). Other raster layers may have bands in which the numerical values correspond to attributes such as elevation, aspect etc. A vector layer consists of a geometry (the shapes and position of things) and an attribute table. Vector geometries can be
There is the additional concept of multiple geometries that we will look at later.
Open street maps (OSM) provide free, open source, geographical information mainly concerned with roads, streets and paths. This is provided both as a base map and in the form of query-able vector layers. These have been loaded onto our PostGIS server, as you will see later.
Both Arc and QGIS are programmable. However most users take advantage of the menu system to avoid learning a computer language. In contrast R is a language. We can use the language to design a reusable script that would order multiple pizzas. If you had a copy of a pizza ordering script designed for Manchester you would not need to know exactly what every line did in order to use it in Bournemouth. You might need to learn a few additional skills in order to adapt it.
We will take the same approach on this course. You will learn to use a point and click interface to design a single analysis for data collected on the field trip. This concerns vegetation types and other spatially explicit data on and around the areas visited and studied. However if the track was produced by a GPS attached to a deer, or multiple deer, you would need to iterate the analyses many times with up dated information as it came in. In this case a script would be needed for a long term deer monitoring project.