Simple Features

In the world of GIS, most of the data we work with is based on the concept of Simple Features. This is a standard (Simple Features Specification) to describe how geometric objects are modelled for use in GIS software.

For us, we can start off thinking of features (remember what features are) as one of three types (feature types) – point, line or polygons (as we reviewed in lecture).

Points

The above image illustrates the basic starting blocks for all features – points. In this example we have points in two dimensional space. As they are a simple model, the geometries can be stored in a text file:

METDATA SECTION
“POSTCODE”,”Postal Code”
“LAT”,”Latitude”
“LON”,”Longitude”
“CTNAME”,”Census tract name”

EVENT THEME SECTION
“POSTCODE”,”LAT”,”LON”,”CTNAME”
“V1H1A1″,53.872327,-122.782974,0004.00
“V1H1A2″,53.876089,-122.783372,0004.00
“V1H1A3″,53.877017,-122.781113,0004.00
“V1H1A4″,53.876796,-122.780557,0004.00

Lines

The above lines are connecting the original points together. The illustration could have five lines (one for each connection) or one line going through each point.

Polygons

The polygon above ensures that if you start at any point on its boundary and continue around it boundary in one direction – you will end up where you started. This polygon could be made up the line(s) that connected the points or drawn as a single polygon connecting the dots in a manner that creates a ring.

Non-Simple Features

If you want to create an ellipsoid, you need to know the mathematical formula that describes its creation

The image above illustrates how you can create an ellipse one one motion following a few parameters of the ellipse formula. This type of feature creation is supported in GIS, but rarely used. Instead, the shape would be a polygon with a plenty points arranged in a shape of an ellipse.

Clean data

The examples above provided nice clean spatial entities or objects (features), but that is not how all spatial data has been collected. We will see this with our GNSS data collection using our mobile phones.