GIS uses many terms that can be confusing to new users, either because they haven't come across them before or because they are used in a less familiar way.
Associated with each feature are one or more attributes holding additional data. These could most easily be understood if the data is considered as a spreadsheet with one row per feature and one column per attribute.
The type of attribute (text, integer or decimal number) and its logical name are defined for a layer.
Attributes may hold information that would determine what is displayed on a map (e.g. the text to display, a colour, angle to rotate a symbol, etc), what features should actually be displayed or other information associated with a feature that isn't shown (e.g. a revision date, the area of a polygon, etc).
CRS or Coordinate Reference System¶
The special small window that opens to allow you to choose options, open or save files, etc.
Feature¶In a vector layer, a feature is a logical element defined by a point, polyline or polygon. Examples of features include:
- Position of a seat - defined by a point, rather like using a push-pin.
- Centre-line of a road - defined by a polyline.
- A lake - defined by a polygon.
Features normally have one or more attributes associated with them that can be used when styling the layer.
A map is typically constructed from overlaid information, somewhat like stacking a collection of transparencies and/or stencils on top of each other. A layer is usually the display of a single file.
Ordnance Survey is the national mapping agency of Great Britain.
A pixel is an individual coloured graphics point in an image, usually square. See Wikipedia.
A plugin is a (usually) small additional program that adds to the functionality of QGIS. These are written in the Python programming language (see Wikipedia.) and are relatively easy to create by someone with moderate programming skills. See Add additional functionality (Plug-ins).
In computer vector graphics, a point marks a specific location in the world. It has no size, but GIS software allows styling to display a symbolic representation on a map - rather like using push-pins, but with much more flexibility.
In computer vector graphics a polygon is an enclosed shape defined by the position of three or more points.
With QGIS, a polygon can be styled in many different ways including outline line width and colour and different fill colours and patterns. See Wikipedia.
In computer vector graphics, straight lines are drawn between two points. More complex lines called polylines join two or more points with straight lines. In QGIS, curves are currently created by linking a number of points - if you zoom in enough you will eventually see the individual straight parts of the curve.
While and enclosed shape can be drawn (e.g. the fence around a field), this isn't the same type of object as a polygon (e.g. the field itself). A polyline has a length but a polygon has an area.
With QGIS, a polyline can be styled in many different ways to change the colour, width and pattern used to represent the line.
Raster or Raster Graphics¶
Scale Dependent Rendering¶
It isn't always appropriate to display all layers at a given level of zoom.
When zooming out, text may be too small to read – with scale dependent rendering you can choose the range of zoom levels appropriate for the layer.
Similarly, you may wish to swap layers at different scales automatically as you zoom in and out.
You can adjust this via the layer Properties->General tab.
Shapefile¶A shapefile is possibly the most commonly supported vector file format and is used both as a layer and as an easy way to exchange data with other users.
Despite the name, a shapefile isn't a single file but is a group of files sharing the same name but with different file extensions. All shapefiles must include the first three files listed below, but other files may be included to extend the functionality of software such as QGIS. Assuming a shapefile called FOO, the individual files could be:
- FOO*.shp* - this defines the layer type (point, line or polygon) and the position of all its points.
- FOO*.prj* - this defines the projection or CRS.
- FOO*.dbf* - this holds the attribute data for the features.
- FOO.shx - this shape index format makes it possible to seek backwards in the shapefile
- FOO.qix - ?
- FOO.qml - if a layer style has been saved with the same name as the shapefile, QGIS will use this as the default style unless subsequently change in the layer properties.