With the advent of computers, electronic files and modern measuring systems, geospatial data is now digital and 3D. In years past, an analogue map stored in a flat file was typically both the end-product of a survey and the storage medium for geospatial information. Now spatial data is stored digitally in electronic files and map users enjoy many more options than those available to users of an analogue map. Of course, paper maps are still used, but these are now generated on demand from data stored in an electronic file and the use or destruction of a paper map does not diminish the value of the spatial data stored in this.

Origins of Datum
Automated processes have enormously enhanced productivity of data collection and map compilation. Maps, paper and otherwise, are now more readily available to everyone than ever before. Further, the use of digital geospatial data has gone beyond the map and now, using web-based software like Google Earth, anyone can view digital geospatial data for any location on Earth from almost any perspective. It seems we’ve reached geospatial Nirvana. From a lay perspective it appears that everything fits together quite nicely, and it does. This is a tribute to human ingenuity and adaptability. But from a technical perspective the digital revolution has created an opportunity yet to be fully realised. Modern measurement systems such as GPS, remote sensing systems and even the electronic total station, all collect digital 3D spatial data. Yet the conceptual spatial-data models used to organise and process measurements are separated into horizontal and vertical components. This is not in itself a problem. The problem is that traditional horizontal and vertical datum has two separate origins: Earth’s centre of mass is the origin for horizontal and the geoid is the origin for vertical. In many cases, geoid and mean sea level are used interchangeably because the geoid closely approximates sea level at rest. However, the reference for vertical is the geoid, not mean sea level.

Mean Sea Level
The tough question is where is the geoid? The geoid is an equipotential surface all points of which are perpendicular to the plumb-line. The number of equipotential surfaces is infinite but the geoid is the one geopotential surface which, in a global sense, best fits mean sea level. The definition is simple and understandable for anyone standing at the coast or on the deck of a ship. In the past, mean sea level was taken to be the average of tide-gauge readings. The Mean Sea Level Datum of 1929 in the US was based upon 26 tide gauges located around the coast of North America. The implication of a mean sea-level datum is that a zero elevation contour staked out on the beach might be used to mark the boundary between what is ocean and what is not. But this is not the case. In order to avoid confusion, on 16th May 1973 the Mean Sea Level Datum of 1929 was renamed the National Geodetic Vertical Datum of 1929. No published elevations were changed: only the name of the datum.

Zero Elevation
In preparation for a readjustment of the vertical control network in the US, loops of very precise levels were run throughout North America. It was shown that the relative internal consistency of the new and existing levelling loops was better than the absolute values provided by the 26 tide-gauge stations. Therefore only one existing benchmark elevation was held (BM Father Point/Rimouski, Quebec, Canada) and all other North American Vertical Datum of 1988 (NAVD88) elevations published with respect to that one elevation. This means the elevation reference surface in North America is arbitrary. Zero elevation is still intended to approximate mean sea level despite the formal dissociation of the vertical datum from mean sea level in 1973. The question now is how closely the NAVD88 zero elevation approximates the geoid and what are the implications of the answer to this? Other relevant questions include which of the quantities in Figure 1 can be determined the most accurately? What is the difference, if any, between the relative and the absolute accuracy of the measured quantity? What quantity does the spatial-data community need and/or use? And is this relative or absolute? Will ellipsoid height ever be adopted for elevation in place of ortho-metric height? And what spatial-data model is most appropriate for use with the previous answers?

Recommendation
Using a spatial-data model having a single origin for 3D data has certain advantages. But the elusive geoid presents an even stronger argument in favour of using the GSDM. The GSDM should be used because it

• provides a consistent 3D model for geospatial data that has a single origin
  
• is compatible with modern technology and digital spatial data
  
• includes a stochastic component for handling error propagation
  
• supports a concise mathematical definition for network accuracy and local accuracy
  
• will allow most spatial-data users to continue performing quality spatial-data manipulations without the need to worry about the subtleties of geoid modelling. Geoid modelling will still be needed and used by those still searching for that elusive geoid.

• Burkholder, E.F., 2004, A 3D Datum for a 3D World, Geospatial Solutions, Vol. 14, No. 5, pp 38-41, May 2006.
  
• Burkholder, E.F., 2002, Elevations and the Global Spatial Data Model (GSDM), Proceedings of the 58th Annual Meeting of the Institute of Navigation, Albuquerque, New Mexico, June 24-26, 2002.
  
• Burkholder, E.F., 1999, Spatial Data Accuracy as Defined by the GSDM, Surveying & Land Information Systems, Vol. 59, No. 1, pp 26-39.
  
• Kleppner, Daniel, 2006, Time too Good to be True, Physics Today, Vol. 59, No. 3, pp 10-11.
  
• NGS, 1986; Geodetic Glossary, National Geodetic Survey, National Ocean Service, National Oceanic and Atmospheric Administration, US Department of Commerce, Washington, D.