GEO-LOGIC Mapping BLOG

Data for digital elevation models (DEM’s) are freely available for download. DEM data is typically a set of georeferenced points with altitude values attached to them. These points are arranged in a fixed grid of maybe 25×25 or 90×90 metres or bigger, depending on the source of the download. DEM data can for example be used to compile contour data. This again allows for example, for visibility analysis in a GIS – this means to indicate areas in a specified radius, that will be visible from a specific point. DEM data will also allow you to perform “aspect” (which wind direction are specific areas facing) and “slope” (steepness) analysis.

Two recommended sources of free DEM data are:

http://www.gdem.aster.ersdac.or.jp/search.jsp

http://srtm.csi.cgiar.org/SELECTION/inputCoord.asp

MapWindowGIS consists of many standard GIS functionalities. However to perform certain specific GIS functions it might be neccasarry to first activate the relevant plug-in to activate the specific functionality. More info about plug-ins can be found here http://www.mapwindow.org/wiki/index.php/MapWindow:Desktop:Plugins

http://www.mapwindow.org/wiki/index.php/GeoreferenceImage

Metadata is specific data about, in this case, GIS data. It can include any of the following:

• Date on which a dataset was compiled
• Name of the person that created it
• Accuracy at which the data was captured
• Geographic extent of the dataset
• Purpose of the dataset
• Coordinate system definition
• Etc, etc.

It is therefor data that explain and describe a GIS dataset, for future reference, by users of the data. This can ensure that a user does not utilise the data for a purpose it is not suitable for, and take any possible imperfections into consideration, when using a specific dataset.

An explanation of basic GIS & GPS features. These are points, lines (also called “polylines”)  and polygons (also called “areas”). Depending on the scale at which the data is visualised, points can represent boreholes, gates, and other features small enough to be represented by a single point, location or coordinate on the ground. Lines can represent roads, pipelines, etc. When a set of lines create a “closed” feature, it can also create, and be referred to as, a polygon. A polygon is a feature for which an area can be calculated.

I recommend the Garmin GPSMAP 76 S or CSx GPS devices for use by farmers and other natural resource managers. Although there might be cheaper devices available, I have vast experience with these two models. I will most probably be critisised for this next comment, but are going to make it anyway: I found the accuracy of both these devices to be better than 3m, at least 80% of the time. This CAN NOT be guaranteed, it is only my personal experience. There are many-many factors influencing the accuracy of any GPS device. One of the most basic factors is “openness” to the sky. The accuracy of better than 3m was obtained be ensuring that the operater’s body dit not obscure the GPS device from the overhead signals (sky), or by making use of an external antenna, mounted on the roof of a vehicle. The GPS-Shop is a good place to start, if you would like to get more information on GPS devices, and are interested in buying online – they are based in South Africa.

If you are a farmer or other natural resources manager, I would like to share the following VERY BASIC guideline: A basic difference between GPS products is the functionality to be able to link the devise to a personal computer with an interface cable (allowing you to download captured waypoints), or not having this functionality. Downloaded data can easily be converted to GIS data for mapping purposes. The same can be done with data from a GPS device without an interface cable, but then data has to be captured by typing it into your computer, after reading waypoint coordinates from the GPS device. For the best online GPS offers in Africa, be sure to visit The GPS-Shop.

Many software programs, such as MapWindow GIS, have plug-in functionality. This means that a user can add a specific function to the software, that might not be required by all other users, and is not made available by default when the application is started.  So, when you are looking to perform a specific task in MapWindow GIS, and do not find a menu item for that task, be sure to look under the “Plug-ins” menu item. The task you are looking for might need to first be selected, before it is activated for use in the software.New plug-ins are continiously being developed for MapWindow GIS and made available with each upgrade. You are also able to search and find new plug-ins, before they are included with a new release, and add them to your current version. Go to http://www.mapwindow.org/wiki/index.php/Plugins if you wish to find out more about MapWindow GIS plug-ins, or would like to check the availability of a plug-in to perform a specific task, not yet available in your current version of MapWindow GIS.

GIS’s use world file information to place a raster image (such as an aerial photograph or topo map) at its correct coordinates, or “correct location on the globe”, also called georeferencing. This world file will have the same file name prefix as the image file itself, but with a file extension such as *.JPW or *.TFW or maybe *.TIFW, depending on the file format of the image file itself. The world file must always be kept together with the image file, in the same directory on your computer. When a georeferenced image file is added to a GIS (such as MapWindow GIS) as a new layer, the GIS automatically reads the coordinate information in the world file. Beware that some image formats such as a GEOTiff (also with file extension *.TIF) or an ERMapper *.ECW image file, can have the world file (georeferencing)  information embedded in the image file itself. This means that there is no seperate world file present, but the image file is still georeferenced. The world file can be read with any text editor. It will typically contain the world coordinates of one of the corners of the image file, as well as the pixel size of your image. These coordinates can be in any one of many available map projections used around the world.

MapWindow GIS is a FREE fully functional open source Geographic Information System available for download here: http://www.mapwindow.org/ . It allows you to read, edit and create GIS data in the ESRI shape file format. It can also utilise GIS grid data and view many different image data formats, such as raster maps and aerial imagery.

Many additional plug-ins are constantly being created and made available, adding valuable functionality to the system. Although online forums are available to get help with using this software, it is often difficult for a newby to get started with this, and most other GIS software.

In future posts we will try to explain some basic functionalities of this software, to help you get started with your own GIS and mapping projects, using MapWindow GIS, as well as other (mostly free) tools and utilities.