Introduction to GIS

A GIS connects what with where

A Geographic Information System collects, stores and manages the location and layout of things (the where) along with a description of those things (the what, and sometimes also who and when). By utilizing computer technology, this connection between what and where allows for powerful analysis and decision support.

The basic building block of a GIS is a feature

A feature¹ is a computer model of a collection of things in the real world that will, for the purposes of the computer model, be identifiable as a single entity. This entity could be a tangible object (e.g., a river) or intangible concept (e.g., a boundary). The computer model of an entity (the feature) contains spatial information (where) as well as descriptive information (what, who, when). The spatial component is stored as a geometry and each item of descriptive information is called an attribute.

Geometry

The geometry of a feature consists of one or more point locations in two or three dimensional space. Each point location is described by coordinates (x, y, z or latitude, longitude, altitude).

A geometry may be a single point or an ordered list of points. If multiple points are connected, meaning all locations along the shortest path between two points are being modeled as an edge of the feature, the geometry is called a polyline, though usually shortened as simply line.

A polyline which is closed (meaning the first and last point have the same coordinates), can be used to model a bounded surface, which is called a polygon.

Geometries can also be more complex in that they may consist of multiple parts. Lines may have breaks and polygons may have additional disjoint parts (separated surfaces that are identified as the same entity). Polygons may also have any number of holes (also called islands).

Feature type

The feature type defines how a geometry is used to model a feature. For example, a closed polyline could be modelling a path (e.g., a circular route) or a surface bounded by the edges formed by the polyline (e.g., a lake).

The feature type tells us whether all locations on the (lake's) surface is part of the model or only the locations along the path (the road around the lake).

Features may also be rectangular entities (geometries with four corners) that reference grid patterns to locations on a map. Such grid patterns and features of this type, are called rasters which is synonymous with “bitmaps”, i.e. rectangular grids of pixels where each pixel represents a different value, typically a colour.

Another type of feature used in PlanetGIS is a rectangular region, in three dimensions, containing a large amount of points obtained from a laser scanner. This is called a point cloud. Individual points in a point cloud are not features because they are not identifiable and cannot have information (attributes) attached to them individually. These points have only coordinates (X, Y, Z), an intensity value obtained from the laser scanner and (usually) a colour value obtained from a photograph taken by the same scanning unit. These points can be used to accurately position features in three dimensions, similar to how features can be derived from images (e.g., satellite photos) in two dimensions.

Lastly, PlanetGIS has a type of feature called web tiles that references images (typically) on an Internet server. The geometry of such features is an entire planetary sphere and the number of each tile corresponds to its location on the sphere.

FeatureId

Each feature is assigned a unique number which is used to link all attributes to the feature in the database. In database terminology, the FeatureId is the primary key.

Attributes

Any piece of information connected to a feature is called an attribute. In the database, each attribute is stored in a different column of a table and all attributes of a feature in the same row (which has the FeatureId as the primary key). The term “attribute” has a double meaning when describing a class of features, where its meaning is closely related to the entire column of a database table.

Features have a numer of fixed attributes, as well as user-defined attributes. The fixed attributes are:

• Easting, Northing, Altitude - the coordinate of the centroid of the feature; corresponds to the geometry of a point
• Measure - a numeric value with no predefined meaning, used to describe a primary measurement or dimension of a feature, e.g., the width of a pipe
• Label - a text value normally displayed along with the feature on a map
• Length - the length of a line feature, or perimeter of a polygon; point features have no length
• Area - the surface area of a polygon; point and line features have no area
• Key1 - a text or numeric value with no predefined meaning, which a user can use to identify a feature or attach further information (which has that value as a primary key)

Feature classes

Features are classified into feature classes (e.g., rivers or roads) which may be further classified into subclasses (e.g., paved roads or dirt roads). The determination of this classification is specific to the needs of the system, but technically, all features of a feature class have the same attributes. Subclasses (optionally) add more attributes to those of their parent class, and always “inherit” the attributes of the parent. For example, features in a feature class for rivers might have an attribute specifying whether they are perennial or not.

A feature class typically has a distinct appearance when displayed on a map, but distinctions based on attached information (attributes) can also be used to create visually distinct “layers” on a map. For example, perennial rivers might be shown as solid lines, while non-perennial rivers are shown as dotted lines. This is called thematic mapping.

Map composition

A GIS map is a combination of a set of layers, arranged in an order such that the minimum number of features are obscured due to overlaps. Each layer is typically a feature class, but can also be a subset of features from a feature class. In PlanetGIS, these layers are called displays and each set of displays is called a view.

The user can control the drawing order, visibility and the minimum and maximum scale at which each display is visible. Each display is assigned a style which specifies the appearance of features on the map. Each style is a collection of symbols, pens, fills & fonts to be used when drawing the geometries of features.

Advantages of using a GIS

Among many other things, a GIS enables you to:

• Capture and store spatial entities and their attributes.
• Visualize spatial data to reveal relationships, patterns & trends.
• Share information in a form that is easily understood.
• Manage updates to your database over time.
• Integrate with existing systems.