PlanetGIS User Guide/Core Concepts
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Contents |
Core Concepts
Introduction
We have touched on a few of the main functions, roughly in the order that a new user will encounter them. To simplify the introductory tutorials, many terms were introduced without much explanation and in some cases looser terms, like “layer” were used. In order to delve deeper into the workings of PlanetGIS, it is now time to get a bit more specific.
What is a feature?
A feature is anything representable on a map that is identified as a single entity. For example the set of lines indicating the centreline of the Nile river is called a feature of the map. Often, exactly what constitutes a single feature is determined by the level of detail of attribute information that is or will be attached to features. One may see a whole street in a town as a feature and simply identify it by its name. On the other hand one may want to attach attributes to each link (a part of the street connecting two intersections), for example for routing applications. One might even need to break a street link mid-block at the entrance of a shopping complex or where a major physical change occur, like the separation of the road by a median barrier.
A feature consists of a unique identifier, one or more ordered lists of coordinates, orientation information and a label.
Types of features are:
- Points
- Lines
- Areas
- Raster images
What is a feature class?
A feature class is a grouping of the same type of features as relating to the information that can be attached and the manner in which they should be displayed. Rivers, roads and countries are examples of feature classes. One might have more detailed distinctions like having perennial rivers separate from non-perennial rivers, motorways separate from residential streets, etc. In practice this boils down to a simple consideration: what does your (non-spatial) database look like. If you have a single table containing information on rivers, it is usually easier to have a single feature class for rivers. (Nothing prevents you from attaching the same table to more than one feature class though...)
In addition to grouping features together, a feature class also contains a host of information that describes itself, its standard appearance on a map, the definitions of links to database tables, thematic displays and perhaps even a schedule for automatically obtaining an updated “snapshot” from an external source.
A feature class can also be a group of sub-classes. If you wanted to keep perennial and non-perennial rivers separate, you could still create a group called “Rivers” to put them in.
In the preceding chapters we have sometimes used the term layer for a feature class. While the simplest word to use without definition, the word “layer” implies an implicit order or a hierarchy, which a feature class does not have. Rivers are just rivers until you decide what you want to show on your map. In PlanetGIS the concept of a display is closer to the CAD concept of a layer.
What is a display?
A display is the application of a feature class – as a layer – in a particular view of a particular map. A display is a reference to a feature class as well as a reference to a display style and other settings for the visual appearance of a feature class, e.g. labelling and scale conditionality, in a particular view of a map.
Displays can also be groups with sub-displays. One might create a group for topographical information, so that you can show or hide a topographical display with one click.
Displays can also reference thematic classifications inside a feature class, instead of the feature class a whole.
What is a view?
A view in a PlanetGIS map is a collection of displays intended to illustrate a particular aspect of a map. Often separate views contain separate themes (in the broader sense than defined below) [todo: find another word?] or different types of data is placed in different views so as not to clutter the map. (Example: a contour map and a cadastral map of the same area).
What is an attribute?
An attribute is a piece of non-spatial information associated with a feature. Attributes are synonymous with fields or columns in a database table. A row of a table contains one or more attributes and is associated with a feature through a defined key, often a primary key.
In PlanetGIS, a table is associated with a feature class through a set of connection parameters and the assignment of the key field. For each feature in the feature class, a row of attributes will be retrieved from the database if a row can be found in which the value of the key field matches the value of the feature's unique identifier.
What is a thematic map?
A thematic map is one in which features are visually styled in distinctive ways according to the classification of an attribute. An attribute may itself be a classification, e.g. soil type, and by using that attribute you can create a map where each soil type is displayed in a distinct colour. Each soil type becomes a theme in the map. Another example is a ranged classification on population density. You might want to specify distinct classifications for population density between 0 and 10/sqm, 10 and 100/sqm, etc. Yet another example is a single theme displaying properties for which municipal taxes are in arrears. A theme like that could be simply flooding property polygons with a red fill and shown on top of all property boundaries.
In PlanetGIS a thematic classification is called a thematic query, because it is so similar to the concept of SQL queries where a set of records are extracted and/or grouped according to some criteria. The result of a thematic query is one or more themes (which here also have some similarities to layers).
Summary
The core concepts described above define the structure of a PlanetGIS project. At the root you have the spatial data and most likely also non-spatial data. The spatial data can be organized into logical groupings as well as classified according to attributes from the non-spatial data.
A map is then built with one or more views, each of which contain a collection of displays. Each display references either a feature class from the spatial database or a classified theme. Each display also controls the appearance of the spatial information it is referencing.
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