Introduction to Web Mapping by Dorman Michael;

Author:Dorman, Michael;
Language: eng
Format: epub
Publisher: CRC Press LLC
Published: 2020-01-21T00:00:00+00:00

7.3 GeoJSON structure

7.3.1 Overview

In this section, we go over the structure of different types of GeoJSON strings you may encounter when working with this format. If you are new to JSON and GeoJSON, it may seem difficult to grasp the exact syntax of the GeoJSON format right from the start. Don’t worry—we will come back to more examples later on in this chapter, as well as in the subsequent chapters. Moreover, rest assured one is almost never required to type GeoJSON strings by hand. Instead, web-map developers generally use pre-existing GeoJSON, exported from layers in other formats using GIS software, or coming from external web resources and databases. As we will see in the Section 7.4, you can create and edit GeoJSON even without GIS software, using web interfaces such as the one called geojson.io. Nevertheless, it is important to be familiar with the general structure of GeoJSON to recognize what type of layer you have at hand, and how to extract meaningful information from it.

GeoJSON is a format for representing Simple Feature3 geometries, possibly along with their non-spatial attributes. The Simple Features standard defines 20+ types of geometry types, and the Well-Known Text (WKT)4 format for representing them5. GeoJSON supports just the seven most commonly used Simple Feature geometry types (Figure 7.1). Having non-spatial attributes is not required for valid GeoJSON, so the seven geometries alone are encountered when representing geometric shapes only, with no attributes. In case non-spatial attributes are present, their combination with a geometry forms a "Feature". Finally, a collection of more than one feature forms a "FeatureCollection". The "FeatureCollection" GeoJSON type most closely corresponds to the meaning of a “layer”, which you may be familiar with from GIS software (e.g., a Shapefile). The hierarchy of GeoJSON types thus includes nine types, which can be grouped into three “levels” of complexity:

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