In+ersec+ion for Spatial People

Understanding Simple Features

Introduction

Simple features are the basic building blocks of a simple vector based GIS. This article provides you with a simple definition of simple features and walks you through the conceptual design of a couple simple feature types.
We have two (2) regular features in Digital Surveying Magazine that deal almost exclusively with vector based GIS. The first of these regular features, entitled “JUMP Into GIS”, showcases the application of open source desktop GIS program OpenJUMP to real world GIS projects. The second of these regular features, entitled “Spatial Super Models”, teaches you how to design simple vector GIS data models, or implementation blueprints.
The information in this article will give you the foundation you need to understand the articles in these two (2) regular features of Digital Surveying Magazine.
We will begin our article by providing you with a basic definition of simple features.

A Basic Definition of Simple Features

What is a simple feature in the context of a discussion about vector GIS?
I have personally found this definition is helpful:
A simple feature is the digital representation of a real-world geographic feature in a GIS.
We can expand our basic definition of a simple feature with this additional information:
A simple feature has two primary elements. The first is a geometry used to represent the shape and location of the geographic feature. The second is a set of basic data attributes that describe the geographic feature’s non-spatial characteristics.
We can further increase our understanding of simple features by considering the Simple Feature Type.

Simple Feature Types

A simple feature type defines a set of requirements that all simple features belonging to that type must obey. They are essentially an organizational tool to help classify simple features.
The most important set of requirements defined by a simple feature type are the data types of the simple feature’s geometry and set of attributes.
These concepts will become more clear with a couple of concrete examples.
Let’s conclude our article with those two (2) examples.

Example #1: Modeling Land Parcels as Simple Features

In our first example we’ll imagine we are building a GIS to manage survey data about land parcels.
What geometry type should our land parcel simple feature type define for land parcel simple features?
A polygon seems like the geometry data type choice that makes the most sense for this simple feature type.
What non-spatial attributes would a land surveyor like to know about a land parcels? What data types would we use to model those attributes?
Most surveyors would be interested in the following attributes:

1) Identity of the most recent vesting grant deed.
2) Tax assessor parcel number.
3) Identity of the written instrument that created the parcel (survey map or deed).

What data types would we use to store these attributes for each simple feature? We would likely use a simple text value or numeric value for each of these attributes, depending on how the values are represented.
Diagram #1 shows a graphical representation of our land parcel simple feature.

Example #2: Road Segment

In our second example we’ll imagine we are building a GIS for a local City road maintenance department.
What geometry type should our land parcel simple feature type define for road segment simple features?
A linear geometry like a simple line segment seems like the geometry data type choice that makes the most sense for this simple feature type. (Depending on the map scale, you might also use polygons to represent the actual footprint of a road segment.)
What non-spatial attributes would a maintenance supervisor like to know about a road segments? What data types would we use to model those attributes?
A road maintenance supervisor would be interested in the following attributes:

1) The width of the physical road surface.
2) The paving material type.
3) The date of the last maintenance activity on the segment.
4) The type of maintenance activity last performed.

What data types would we use to store these attributes for each simple feature? We would likely use a simple numeric value for the width attribute, simple text value for the paving material type, a date data type for the third attribute, and a simple text value to identify the type of the maintenance activity last performed.
Diagram #2 shows a graphical representation of our land parcel simple feature.

Conclusion

This article provided you with a basic definition of simple features, explained the requirements put into place by a simple feature type, and looked at two (2) examples of how simple features could be designed.
Future articles in the Digital Surveying Magazine newsletter will look in more depth and simple features, their design, and their role in a GIS.

This article was shared with Slashgeo in a media partnership with Digital Surveying Magazine.