Practitioner’s Forum Revisited

In this issue of the Journal of Surveying Engineering, the section Practitioner’s Forum is revived after several years of complacent hiatus. The broadening universe of the Global Navigation Satellite System (GNSS) has radically changed the way we comprehend and practice geodesy and surveying today. Nobody questions anymore the advancements that the Global Positioning System (GPS) has brought upon many scientific disciplines in general, and surveying and mapping in particular. However, on certain themes, primarily those concerning the definition of vertical control and height systems, geodesists are still debating the true benefits of GPS. Many of them are reticent to change their traditional ways of thinking. They continue to assign more intellectual weight to theoretical physical concepts than to unsophisticated practical solutions strictly driven by geometric principles when restricted, this should be emphasized, to conventional engineering projects of limited surface extend.

Considering that surveying engineering primarily concentrates on localized three-dimensional domains, perhaps some simplifications could be introduced without resorting to a rigorous theoretical development based on mathematical geopotential theory. This may be unavoidable when the earth’s gravitational field and its effects are studied over large land masses. However, it is well known that surveying engineering is concerned with very specific operational activities such as construction of roads, buildings, airports, tunnels, dams, bridges, water supply, sewage systems, etc. that are generally confined to reduced topographic spaces. In the majority of such applications, the assumption of an ellipsoidal earth as the surface of reference for a local vertical datum may be a sufficient approximation.

The fundamental premise for this kind of practitioner’s forum, as previously pointed out by several editors of the Journal, who preceded me (Earl Burkholder, Robert Burtch), is exposing the readers to straightforward practical implementations of seemingly complicated subjects. The relationship and utilization of orthometric versus ellipsoid heights is the example at hand.

In the current issue is presented a well-known geodesist’s personal perspective, that at first glance may sound polemic in nature, but after paused reflection, could provide future practical GNSS solutions to old cumbersome height determination problems. The main question we face is whether Dr. Kumar’s proposed alternative—not original, as he honestly recognizes—that GPS ellipsoid heights could replace orthometric heights on small construction engineering surveys is feasible and adequate at the required level of accuracy. Perhaps, in countries such as Bolivia, Nepal, etc. that are starving for good heights, simple-to-obtain relative ellipsoid heights could be greatly advantageous for certain applications.

However, we prefer to restrict our study to practical engineering situations. Therefore, we invite the submission of manuscripts as Technical Notes or Practitioner’s Forums from all civil engineers in the field of surveying providing alternative scientific evidence that unequivocally supports contradicts the assertions made by Dr. Kumar in this issue’s Practitioner’s Forum. The applications and the methodologies that are investigated in this context should be restricted to surveying engineering projects where a dual set of observations, leveling and GNSS, are available to fully analyze and report on the validity of using ellipsoidal heights for most day-to-day surveying engineering operations.

One important point to keep in mind is that the variables to be compared are the differences between ellipsoid heights $\left(\Delta h\right)$ and orthometric heights $\left(\Delta H\right)$ denoted here following conventional nomenclature. Obviously, for accuracy, the $\Delta H$ values should be directly determined by precise spirit leveling without the aid of a geoid model. The intent is to know how well the differences $\Delta h$ and $\Delta H$ at common points compare when contrasted against different engineering surveying environments using the most current GNSS techniques such as static relative GPS, RTK, virtual CORS networks, etc. On the other hand, differential leveling should follow long-accepted methodology practices. These comparisons could be done one-on-one and∕or after a statistical adjustment. As always, the submitted contributions must be original and must surpass the Journal’s, standard peer review process.

The implications of being able to replace $\Delta H$ by $\Delta h$ in engineering applications could be momentous. The amount of time spent doing traditional accurate leveling could be considerably abbreviated and, consequently, the operational budget for the surveying layout drastically reduced.

Paraphrasing Dr. Kumar’s own words “a very important point to note is that in extremely flat areas, the difference between $\Delta h$ and $\Delta H$ was practically zero. Thus, an engineer will be able to use ellipsoidal heights with confidence.” He has based this assertion on results obtained using real data sets from the NGS database. Any rigorous investigation detailing the pros and cons of this statement in connection with engineering applications is welcome. Personally, I am quite convinced of Dr. Kumar’s assertions, but other independent scientific research must corroborate it. Therefore, the only remaining alternative is to prove that this simplification is applicable to part or all surveying engineering undertakings when GNSS methods are employed. Thus, once more, the Journal reopens its pages to a new Practitioner’s Forum targeted to further increase our body of knowledge about one topic closely connected to the essence of the surveying engineering profession. So the “Practitioner’s Forum” tradition goes on, seeking to bridge the gap between theoretical postulates and practical implementation.