NRSC 2230 / GEOG 2750



Datums tie our coordinate systems onto the physical surface of the earth.  As you would have read earlier, our coordinate systems are based on theoretical representations of the earth, the spheroid / ellipsoid being the most useful.  The datum provides a frame of reference for making measurements on the spheroid.

Learning Outcomes

Upon successful completion of this topic you would be able to:

  • Explain the concept of a Datum
  • Describe the components of the NAD27 and NAD83 datums
  • Explain the problem of using data captured using different datums

What is a Datum

A Datum is an exact specification of a geodetic coordinate system relative to the physical earth components.  This specification is expressed in terms of:

  • the location of its origin.
  • the orientation of the axes.
  • the geometry of the reference surface.


Some datums have their origins near the centre of the earth, while others have their origin at a known point on the surface of the earth.


Some datums express the orientation of their axes as a direction to another point and others use the directions of the X,Y and Z axes of the reference surface.


This expressed as a radius if a sphere is used or the size and shape of the spheroid if that surface is used.

Activity 1 – Finding more about Datums, NAD83 and NAD27

Please use the links below to get more information on Datums. The links provided on the second link give useful information on the North American Datums (NAD27 & NAD83) that we use in British Columbia.  This information will help in completing Activity 2 below.



Activity 2 – Discussing NAD27 and NAD83

Write a brief posting in your blog describing the specifications of the NAD27 and NAD83 datums.  Also explain why data based on one datum cannot be used with data based on the other.  Please provide links to any other useful sites you may have visited.  Name this posting “Datums we use”.

Activity 3 – Comment on “Datums we use”

Post a comment on at least two of your classmates’ posting on datums.