Control Datums, Realizations, and Epochs

Overview

Understanding geodetic datums and their realizations is fundamental to modern surveying. California surveyors must work with multiple datum versions and understand how coordinate values change between them.

Learning Objectives

• Understand the concept of geodetic datums
• Distinguish between horizontal and vertical datums
• Work with datum realizations and epochs
• Convert coordinates between datums

Key Concepts

Geodetic Datums

A datum consists of:

• An ellipsoid (mathematical model of Earth's shape)
• Its orientation and position in space
• A network of control points

North American Datum of 1983 (NAD 83)

The official horizontal datum for the United States:

• Based on the GRS 80 ellipsoid
• Originally realized in 1986
• Multiple subsequent realizations (adjustments)
• California uses NAD 83(2011) epoch 2010.00

NAD 83 Realizations

Progressive improvements to NAD 83:

• NAD 83(1986): Original realization
• NAD 83(HARN): High Accuracy Reference Network
• NAD 83(CORS96): CORS-based adjustment
• NAD 83(2011): Current realization, epoch 2010.00

Vertical Datums

Primary vertical reference systems:

• NAVD 88: North American Vertical Datum of 1988
• NGVD 29: National Geodetic Vertical Datum of 1929 (superseded)
• Separation between datums varies by location

Epochs and Velocities

Modern coordinates change over time:

• Plate tectonics cause continuous movement
• Epoch specifies the date of coordinates
• Velocities describe rate of change
• California moves approximately 46 mm/year

Key Terms

• Datum: Reference system for determining positions
• Realization: Specific implementation of a datum
• Epoch: Reference date for coordinate values

Practice Questions

1. What is the difference between a datum and a realization?
2. Why is epoch important for California coordinates?

Based on CalTrans LS/LSIT Video Exam Preparation Course materials.