The Nigerian National Datum: The Call for Further Realization

The Nigerian National Datum: The Call for Further Realization

Fesowola O. V. Akintoye mnis, rs, mSCgis, MSc GIS & Environment.
Email: fesowola@glogeomaticsnigeria.com.
 

Modupe Adeniyi anis, rs, BSc (Surveying and Mapping Sciences), 

MSc (Geodesy)
Email: modupe1@hotmail.com

 The Minna datum, a classical (optical) reference system have been discussed severally in the past years but still being used as basis for the Universal Transverse Mercator grid coordinates system have been over criticized for its various limitations. The datum was the basis of the Nigerian National Coordinates System which is no more in use. Both the national coordinates system and Minna datum are our joint mapping legacies upon which our national heritage has been built. A future without the understanding of the past and learning to overcome the limitations of the past could be dangerous. Every nation has its mapping identity and interface for global comparison. The surveyors in Nigeria have a rich heritage in the system and datum.

The problems with classical methods used for several legacy datum across the globe has been resolved by breaking down national framework into state level frameworks, carrying out state level adjustments and re-adjustments at the national grid level. This is being further improved and strengthened by connecting to ITRF frameworks. Just as the ITRF framework is being further improved on, every legacy framework requires the same efforts.

The International Terrestrial Reference Framework (ITRF) which are realized from sets of coordinates of some points located globally and are managed by International GNSS Service (IGS) on which GNSS CORS stations are being operated is being improved on continually due to improvement in processing strategies, techniques, and self-consistent reprocessed solution.  There are different realizations since 1988 with the current realization adopted and in use, the ITRF2008 at epoch 2005.

The coordinates or position of points derived using GNSS data acquired in 1997, ITRF 1997 realization will differ from that in result using ITRF 2005 and ITRF 2008 realization, not to talk of ITRF2013 which would be adopted before the end of the year. Each new framework provides a better understanding and capability for better determination of points on the earth. How do we respond to the management of these changing and disparate coordinates of positions?

It is a fact that legacy and some current geospatial data are likely to be less accurate than what we are collecting today and this raises the question of how to make good quality decisions when the data is so disparate. For example the American NAD 83 and WGS 84 have the same datum but differ by about 2 meters. The NAD 83 datum origin was built on optical measurements like our Minna datum. It has been improved upon because every datum must be realized, must be connected to the earth; for example to the GNSS frameworks to improve the accuracies. A single datum can have multiple realization and for example, NAD 83 and WGS 84 have had 4-5 realization. For better understanding of how legacy datum can be improved and adopted for national relevance, the NAD 83, similar to Minna datum is worth looking at.

NAD 83

- Original realization was completed in 1986, almost entirely classical (optical) observations like the Minna datum.

For improvement, the datum has been improved on thrice

·       High accuracy reference network was established by the establishments of state by state network framework based on GNSS but included classical stations.

·         This was followed by two national re-adjustments of

o   2007

*  NAD 83 (NSR 2007/CORS 96) Epoch 2002.00

*  National Adjustment (GNSS only)

o   2011

*  NAD 83(2011/PA11/MA11) Epoch 2010.00 that included a nationwide GNSS only adjustments and a realization of same for CORS and passive stations.

According to NGS, the result is not a new datum but still NAD 83 and the national adjustment includes 80,872 passive control stations and 5 networks.

To ascertain the changes in positional coordinates and improvement in ITRF framework at newer epochs, The UNEC station was processed using the Trimble Online Processing facility to reflect the disparities in the coordinates between 2011 and 2013 using ITRF 2000, 2005 and 2008 at epochs 2000, 2005 and at epoch of data respectively.

Figure 1: Coordinates of UNEC using different ITRF Realisation

Figure 2: Position Differences in ITRF 2000 at Epoch 1997 and ITRF 2008 at Epoch of the data

Figure 3: Position Difference of UNEC using ITRF 2005 at Epoch 2000 and ITRF 2008 at Epoch of data

Figure 4: Position Difference of UNEC using ITRF 2008 at Epoch of data and ITRF 2008 at Epoch 2005

 Chart 1: Chart showing 2D Positional Error for UNEC Station using different ITRF Realisation

From above, it revealed that we can have positional error between 0.2m and 0.5m if the current ITRF realisation and epoch of the GPS data are not taken into consideration for our GPS base station. The error budget can be more, depending on duration of observation, processing strategy (IGS products http://igs.org/products), and GNSS post-processing software (Scientific or Commercial) available for post-processing the data.

There is need to determine the change of point coordinates as a function of time due to plate tectonic movements. This is dependent on the epoch at which the coordinate was determined. From figures 2, 3, and 4 it is evident that there is a minimum of 20mm position movement due to plate tectonics, which if not taken into account over a long period of time would affect the quality of station coordinates. The online plate motion calculator and the results in figures above closely agree as shown in figure 5.  http://www.unavco.org/software/geodetic-utilities/plate-motion-calculator/plate-motion-calculator.html

Figure 5: Result from online plate motion calculator for UNEC Site

The various realizations of the ITRF framework bring us closer to precise positional measurements and fixes and better understanding of how tectonic plates movements and velocity improves our work. It is a fact that both legacy and new geodetic datum(s) get better over time gradually achieving their realization. The geodetic framework that produced the Minna datum was limited in space and technology; and therefore the need for expansion, harmonization and connection to the GNSS grids for better accuracies.

There is a need to go back to the drawing board, revisit previous works, re-awaken the interest of those who worked tirelessly in the past on the growth of the datum and the NNO; and pick up our abandoned legacy in geodesy, else all efforts by the Fajemirokuns, the Agajelus, other researchers and recent efforts by OSGOF and NSMDA will amount to wasteful ventures.

Every geodetic framework has been the dream and vision of a generation, most of the realization were achieved by newer minds and researchers in the following generations; the research and contributions to Minna datum and the Nigerian National Coordinates System have been enormous and fruitful so far. Just as efforts by the global community have helped in the realization of the global datum(s) and giving us a better knowledge of the magnitudes of the velocity and movement of the tectonic plates at global scales; a local datum can help in providing a larger scale understanding of tectonic plate movements. The time to work on the datum and re-design the NNO to correct for the various errors in them is now.

The Minna datum is a dream we must realize and no one will do it for us if we don’t make efforts and help will only come when we are seen committed and working at it. The datum will provide indigenous framework that can be translated at will into any required global framework. It must be realized and connected to the earth for quality and effective use.

There has been problems over the use of different datum(s) with the UTM in Nigeria, or cross usage of data acquired at different ITRF realization and epochs; therefore, Surveyors and GIS professionals need to assess coordinates of datasets that are provided by various agencies using GNSS techniques to ascertain that there are no disparities in the coordinate reference system used.  The use of the phrase that “coordinates are based on WGS84” is an old fashioned way of supplying point coordinates. It is highly recommended that the realization epoch must be stated for users of the information to determine if there is need for transformation from one epoch to another (ftp://itrf.ensg.ign.fr/pub/itrf/WGS84.TXT).