3D point features
What are 3D point features?
A point feature is a GIS object that stores its geographic representation—an X and Y coordinate pair—as one of its properties (or fields) in the row in the database. Some point features—such as airplane locations—need to also include a z-value, or height, to correctly locate itself in 3D space. These objects must be modeled as 3D point features.
3D point features embed their z-coordinates inside the geometry, or Shape field, of their feature class. This means that z-values are automatically included with every new point feature, allowing it to represent any 3D position in space—regardless of whether that point is on, above, or below the ground.
Examples of 3D point features include:
- locations of aircraft
- subsurface seismic points
- address points inside skyscrapers
- underground subway stations, and
- locations on top of buildings (such as counter-sniper units)
While it is also possible to model z-values using a numeric feature attribute, this option may not support all the same analysis and interaction options supported with embedded z-values.
Any features that live on the elevation surface, such as street furniture, vehicles, and trees, should always be modeled as 2D point features. This simplifies the data storage and maintenance of the features, and allows them to automatically adjust their heights if the underlying surface data improves. Also, if z-values are ever needed for these points, they can be calculated on to the features by referencing the elevation data and using tools such as Interpolate Shape.
Two ways to create a 3D point feature class
There are two main ways to create a 3D point feature class: you can either create a brand new point feature class or you can convert existing 2D point data into a new feature class that contains z-values.
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To create a new 3D point feature class: |
Check the "Coordinates include Z values" check box when defining the feature class's geometry. |
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To convert existing 2D data, there are multiple options available to set where the z-values come from, including: |
Setting the height values from an elevation surface or using an existing feature attribute. |
What should the z-value represent?
Z-values are primarily used to include elevation in your GIS features. The values can represent absolute heights, such as an aircraft at 30,000 feet, or relative-to-ground heights, such as a subway station 50 meters below the surface of the ground. Both methods are fully supported in the display and analysis of the resulting 3D feature class.
It is worth noting that z-values can also be used for including other vertical measures such as air pollution observations, temperature, and other measures that are used in surface generation. Creative use of z-values, and what they represent, can be very useful in the understanding and analysis of your data.
The units and datum for a feature class's z-values should be defined on the containing feature dataset (if one exists) or on the feature class itself (if there is no feature dataset). If no units are defined, ArcGIS will assume that the Z units match the XY units. This assumption can be problematic, particularly if XY units are geographic (lat/long).
Creating 3D point features
3D features can be created interactively, using the standard ArcGIS editing framework in ArcMap, ArcGlobe, and ArcScene, or through the geoprocessing framework, using tools that incorporate z-values in the output.