3-D: A Challenge for Surveying in the 21st Century

© 1999 by Earl F. Burkholder, PS, PE

Global COGO, Inc. - Las Cruces, NM 88003

Imminent arrival of a new millennium is used as an excuse to review how surveying practices have been affected by technological developments during the past 50 years and to raise questions about more changes as we look to the year 2000 and beyond. With modern equipment, collection of spatial data (surveying measurements) is less expensive now than it was, say a decade ago. And, electronic media makes it possible to store enormous quantities of digital spatial data quite economically. In part, because spatial data are available and/or inexpensive, spatial data applications based upon geographic information systems (GIS's) are now found in many disciplines.

We live in a three-dimensional (3-D) world. Spatial data are collected using 3-D tools such as global positioning systems (GPS), photogrammetric mapping, and total-station instruments. Spatial data are routinely displayed and plotted as maps or 3-D renderings by various data visualization routines. But, traditional models used by surveyors and mappers for handling spatial data make a distinction between horizontal and vertical. There are horizontal datums and vertical datums, each with its corresponding database. As we look into the next century, I see those two being combined into a single 3-D datum. The global spatial data model (GSDM) makes it possible to integrate horizontal and vertical concepts into a single model which stores 3-D spatial data in a single database and which supports accuracy considerations at any level while allowing each user the option of working with horizontal components, vertical components, or both. A challenge for the surveying profession as we enter the next millennium is to consider the impact of adopting a GSDM and to provide leadership in developing coherent standards, specifications, and procedures which will be valuable to all spatial data users regardless of discipline. For field surveyors, the "extra" procedure is remembering to record the height-of-instrument (HI) of each total-station/reflector measurement.

Surveying practice in the 21st century will be very different than it was when I started working as a draftsman in 1968. The firm I worked for then was very progressive and had a desk-top computer (about the same size as today's PC) which would compute a sine or cosine function in less than 11 seconds. Given the data, instructions, and the computer, one could compute and adjust a traverse before lunch (a task now measured in minutes or even seconds). Old-timers berated me for not appreciating the time it took for them to look up trigonometric functions in the tables or to perform surveying calculations using logarithms. Admittedly, I was guilty as charged. I just couldn't get excited about learning boring tedious manual computational processes. Come to think of it, students today are not that much different. They too like learning and using new technologies.

During the growth and development of our country (from about 1800 to about 1950), surveying practices in the USA changed rather slowly. Yes, railroads were built from coast to coast, the US Public Land Survey System was laid out, and standard surveying practice evolved from the compass and chain to the transit and steel tape. But, except for the geodetic surveyor working for a federal agency, most surveyors performed computations using latitudes and departures and maps were drawn by hand or plotted in the field using a plane table.

However, during the 1950's, 60's, and 70's, technological developments contributed to significant changes in the surveying profession and, as we look ahead to the 21st century, it appears even more changes are on the way.

The rate of change has accelerated even more during the 1980's and 1990's with implementation of three new technologies; 1) the personal computer (PC), 2) the global positioning system (GPS), and 3) the world wide web (WWW). These technologies are revolutionizing the surveying profession and the services surveyors are called upon to provide society. According to Marc Cheves, Editor of the Professional Surveyor magazine, in the editorial of the January/February 1999 issue, the current buzzword for surveyors is "databases." He states that, "we have only begun to see the importance of databases in our work and lives." He also goes on to describe a shortcoming of most GIS databases - they are only 2-D. But, he notes that most software vendors are adding or have added the Z component to their software with the implication that that is what it takes to work in 3-D.

That may be true if one understands how horizontal and vertical data have been combined and if one is aware of the associated limitations. For example, the use of map projections (state plane coordinates) should be examined carefully before investing in a 3-D database because the map projection is strictly a two-dimensional mathematical model. That means a separate model, typically elevation above sea level, must be used for the third dimension. Some GIS software packages use elevation as an attribute of location and the system has been referred to as 2.5-D. Other software packages simply use elevation as Z, the third component for rectangular x/y/z coordinates. Such a 3-D system is called pseudo 3-D by this author because the reference for the third dimension is a curved surface, not the flat x/y plane. However, the pseudo 3-D system has been used extensively and works just fine so long as one recognizes its limitation. With the advent of GPS, a third option is to use the ellipsoid height model for the third dimension. In all three cases, it is inappropriate to use rules of solid rectangular geometry beyond the limit one can safely assume a flat earth.

The January 1999 issue of CE News contains a list of 16 software vendors who offer coordinate geometry (COGO) and digital terrain modeling (DTM) software. In all but two cases, 3-D capability is either stated explicitly or implied by the intended use (contouring/cutsheets/earthwork/volume). But, none of the vendors advertise true integration of horizontal and vertical into a single 3-D database such as that supported by the global spatial data model. One reason is that users haven't asked for it and/or demonstrated a willingness to pay for it. Equations for the GSDM are widely known and some software packages already support true 3-D capability but, until clients ask and/or pay for it, vendors seem reluctant to push true 3-D as a selling feature. Bringing appropriate software tools to market is another part of the challenge.

Just what is the GSDM? Interested readers are encouraged to read the following three articles by the author on the GSDM published in the Professional Surveyor magazine:

Two other articles by the author which describe the GSDM on a higher level are: More articles on the GSDM.

In conclusion, the global spatial data model has been defined, reviewed by high-level professionals, and not found wanting. It accommodates 3-D spatial data, digital data storage, modern measurement technologies, solid geometry and vector algebra equations throughout, a user defined perspective of any 3-D data set, and any level of spatial accuracy separately in each of three dimensions. Additionally, the GSDM has no zones or projection constants, uses one set of equations world-wide, and provides local ground level horizontal distances and true 3-D azimuths between points. Furthermore, it does not distort physical measurements as does the 2-D map projection model. So far, the primary obstacle to using the GSDM seems to be "we don't do it that way." Maybe that's the biggest challenge of all.

Read a better mouse trap description of the GSDM.

Read the anecdotal pan story about using an appropriate 3-D model.

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