ArcGIS Tutorial: Basic knowledge of 3D data

The definition of a three-dimensional GIS data (x, Y, z) contains an extra dimension, the value of which. Z-values have units of measure that can store and display more information than traditional 2D GIS data (x, y). Although the z-value is usually the actual elevation value (such as altitude or geographic depth), there is no requirement that this method be enforced. Z-values can be used to represent many things, such as the concentration of chemicals, the suitability of locations, or even the values that represent ratings.

There are two basic types of 3D GIS data: Feature data and surface data.

　　3D feature Data

Feature data represents discrete objects, and the 3D information for each object is stored in the geometry of the feature.

Three-dimensional feature data can potentially support multiple different Z-values for each x, y location. For example, a vertical line has one upper vertex and one lower vertex, and the 2D coordinates of the two vertices are the same, but the z values are different. Another example of 3D feature data is a 3D multipatch building whose roofs, interior floors, and foundations all contain the same 2D coordinates, but with different Z-values. For other 3D feature data such as the 3D location of the aircraft or the uphill walk path, each x, y location corresponds to only one z-value.

　　Surface data

Surface data represents the height value above an area, and the 3D information for each location in the area can be stored as a cell value or inferred from a triangular network of 3D polygons.

Surface data is sometimes referred to as 2.5D data because it supports only one z-value for each x, y location. For example, the elevation of the Earth's surface will return only one value.

　　When to model GIS data in 3D

Creating and maintaining 3D GIS data is more difficult than 2D data, so only a three-dimensional modeling of data is necessary if additional effort is helpful to your work. Some GIS features (such as aircraft locations or underground wells) themselves need to be 3D modeled, while other data may have the same effect at 2D and 3D. For example, 3D modeling of a road network may be useful for investigating gradients, but the extra effort to maintain Z-values may well outweigh the benefits he brings.

When deciding whether to model your data 3D, consider the following considerations:

• GIS data does not have to be 3D modeled or displayed in a 3D view.
• When necessary, you can easily add a surface's height value to a 2D object by using a geoprocessing tool.
• If the source of the z-value is a surface, consider how often the base surface changes. The more times it changes, the less help it can have on storing Z-values for the generated features.

If you decide to model some or all of your data in three dimensions, the most important thing is to determine the units of Z-values. When you start editing and maintaining Z-values, it's important to understand what Z-values represent. One basic rule that should be followed as much as possible is that the Z-units should match the X, y units. For example, if the data is in a (meter-based) UTM region, the z-values should be modeled in meters. This helps you interact with the data in an intuitive way, for example, when measuring 3D distances or moving objects by changing x, y, and Z coordinates.

ArcGIS Tutorial: Basic knowledge of 3D data