concept of Geodetic Controls Network in Dam Structures and their Under utilization in the Northern ’s dams.

Abdulkadir, Isah Funtua Msc. (Nig), mnis. and Mutari A. K. Department of Surveying and Geoinformatics Abubakar Tafawa Balewa University Bauchi.

Abstract This paper reviewed the concept of geodetic controls network, their importance and application in monitoring of Dams Structures within the purview of Engineering Geodesy. It highlights the underutilization of these control networks in the monitoring systems of Dams structures in the Northern Nigeria. The paper advocated for strategic manpower development through geospatial education and adequate provision of control points and instruments towards a regular status monitoring of the Structures which shall involve the full utilization of Geodetic Controls Network which was hitherto neglected. Key Words: Geodetic Controls, Dam deformation, Dam Monitoring System, 1.0 INTRODUCTION The need to monitor dams for detection of any possibility of dam failure and flooding has been of great concern to the government and the various communities that lives around the localities where such structures were sited and as well as the downstream communities. For effective control and monitoring, during and after construction, of dam structures, precise control points are normally established. These points constitute the main frame used for detection of any variation arising, such as shift due to deformation, stress and shear of the structures. The points are Locations with established coordinates; latitude and longitude, and often elevation, used for accuracy and precise location of other points and constitute the main frame on which less precise observations may be based. They are as important as planning, design, and execution and “as built”. There are many historical cases of dam failures where early warning signs of failure might have been detected if a good dam safety-monitoring program had been in place. The monitoring program provides the information that is needed to develop a better understanding of the on-going performance of the dam (Barry and Statelier 2010). Sadly however over the past decades, in the Northern Nigeria, despites various instances serious economic and human lives loses due to the absence or ill timed responses by the management of these structures, there is no evidence of the existence of, or of any efforts towards the establishment of an articulated Dam safety monitoring program particularly in the North part of the country. This paper reviewed the significances of the application of geodetic controls in Dams structures monitoring system The objectives is to advance the course for inculcating the culture of short and long term monitoring of Dam structure that include the Geodetic control usage in monitoring the behavior of the structures during the construction period and all along the operation time. Common causes of dam failure include overtopping, foundation problems, structural problems, and piping (internal erosion due to seepage). With an effective monitoring program, these causes can be detected early and repaired or mitigated. There is always an associated high accident risk potential of significant proportions not only for the dam structures themselves, but also for the population living in the area involved as witnessed in the recent year 2012 flood episode in Nigeria principally along River Benue and down stream of River Niger as a results of opening of a gate of a Dam Upstream River Benue in Cameroun Republic. 2.0 THE CONCEPT OF GEODETIC CONTROL The geodetic control consists of geodetic control stations and their related information – the name, feature identification code, latitude and longitude, orthometric height, ellipsoid height, and metadata for each station. The metadata for each geodetic control point contains descriptive data, positional accuracy, condition, and other pertinent characteristics for that point. According to the US Federal Geographic Data Committee, [Part 4: Geodetic Control (FGDC-STD-014.4-2008)] “Geodetic control provides a common reference system for establishing the coordinate positions of all geographic data. It provides the means for tying all geographic features to common, nationally used horizontal and vertical coordinate systems. Therefore fundamental geodetic networks of horizontal and vertical control provide asset in the form of fixed homogeneous coordinate reference systems which forms the basis of all spatially related information. The main features of geodetic control information are geodetic control stations. These monumental points (or in some cases active Global Positioning System control stations) have precisely measured horizontal or vertical locations and are used as a basis for determining the positions of other points. . Geodetic control information plays a crucial role in developing all framework data and users’ applications data, because it provides the spatial reference source to register all other spatial data. In addition, geodetic control information may be used to plan surveys, assess data quality, plan data collection and conversion, and fit new areas of data into existing coverages.” The horizontal control is normally provided through Triangulation, Trilateration, and Traversing method while the vertical control will normally be provided by differential leveling, trigonometrical heighting, and Inertial Survey systems or by other techniques. 3.0 DAM MONITORING Dam monitoring relies on the long-term measurement of small structural motions at regular intervals. Traditional surveying techniques and geotechnical instrumentation can effectively monitor one- or two- dimensional modes of these motions. (Stewart and Tsakiri 2001) The monitoring of the movement of dams demands geodetic accuracy over relative short distances and this falls within the purview of Engineering or miniature geodesy. This class of geodesy employs many of the Instruments and practices of normal geodesy and requires exceptionally precise centering to an accuracy of 0.1mm (Bomford 1980). For any structure, monitoring has to ensure the longevity of life and safety of the structure. In the case of the dam structure, it must enable the timely detection of any behavior that could deteriorate the dam, potentially result in its shutdown or failure, in order to implement corrective measures. Therefore the main purpose of dam monitoring is to study whether or not the dam is behaving according to design predictions and to verify design assumption. Monitoring parameters such as leakage pore water pressure, and deformation can provide an indication of the performance of the structure. Consequently a good dam safety monitoring program should be a key part of every dam risk management program. Preventive actions are directly taken by means of installing devices that are able to monitor the behavior of the dam structures. These instruments should be capable of measuring the magnitudes involved, with the adequate precision, for further comparative analysis with the mathematical models provided in the design. According to Barry and Statelier (2010), the scope of the monitoring methods employed depends on the potential risk associated with dam and site characteristics. Such characteristics include:

i. Dam height and type ii. Extent of potential damage to people and structure located in flood zone iii. Reservoir and spillway capacity iv. Site seismicity Foundation weakness zones The measurement techniques are generally divided into geodetic and geotechnical/structural methods. a) Geodetic method: conventional & terrestrial survey and space-based method. Global information. Geodetic measuring devices measure georeferenced displacements or movements in one, two or three dimensions. b) Geotechnical/structural: Geotechnical measuring devices measure non-georeferenced displacements or movements and related environmental effects or conditions. 4.0 DAM MONITORING SYSTEM All dams are required to have a level of instrumentation that enables proper monitoring and evaluation of the structure under all operating conditions. Therefore control of a dam requires a wide range of important information coming from the sensors, which are of vital importance for the life of a dam. A specific and sudden decision for correct control of the reservoir, of the dam body behavior and of foundations is often taken. Monitoring is not only carried out by sensors but also involves direct or remote visual inspection as well as topographical measuring. According to Barry and Statelier (2010) the three critical components of a dam monitoring system are: 1) Instrumentation, 2) Data collection, and 3) Data management. Currently, there are many tools available for making improvements in these three areas an optimizing a dam’s monitoring system. The level of technology applicable for a particular dam project varies greatly depending upon the objectives of the monitoring system. 1. Instrumentation : Geotechnical instrumentation at dams consists of piezometers, seismic strong motion instruments, crest survey monuments, tilt plates, inclinometers, and other instruments as deemed necessary to adequately monitor embankment performance. Automated instrumentation systems are currently being installed at each dam to collect and transmit data from the piezometers, seepage measurement devices, pool level sensors and strong motion instrument. The instrumentation are also categorized as civil instrumentation and geodetic instrumentation. The civil instrumentation includes Extensometers, Clinometers, and Plumb lines, among others. The geodetic instrumentation includes GPS, total stations and levels. The two types are applied as a methodology for comparing the measurements among different periods of observation, determining the differences, that is, possible displacements. Geodetic instrumentation has characteristics that complement the observations conducted with civil instrumentation, as the reference for the measurements is absolute (stable) and not related to the previous measurements. Thus, the displacements determined in function of the measurements carried out by a geodetic instrument always have the same absolute reference. However, civil instrumentation, a direct plumb line (PL), for example, stores displacement measurements related to its installation, which along time turns to be the reference for further measurements. 2. Data collection, There are three general methods of data collection commonly being employed .These are 1) manually reading the instrumentation and recording the values on paper, 2) Manual or electronic readings that are recorded on handheld computers in the field, and 3) automated data acquisition systems. The data collection scheme must provide built in levels of accuracy and reliability to ensured acceptance of the raw data. Information on piezometer levels and other geotechnical parameters has traditionally been collected by hand or by ground-based radio or hard-wire telemetry systems .telemetry networks combining line-of-site radio, hard-wire, and recently GOES (Geostationary Operational Environmental Satellite) messaging 3. Data management During data processing stage, raw survey data must be converted into meaningful engineering values. The procedures for data reduction should be based on the most rigorous formula and data processing techniques. A least square adjustment technique is recommended for data processing. The data management should be based on an information management using computer based method. This is to enable ease of archiving, indexing and cross referencing to the existing structural performance records. 5.0 DAM DEFORMATION Deformation is the major source of dam failure. This may due a number of causes that occurs in 1) Concrete structures 2) Embracement structures, or due to 3) general structural distress that includes foundation problems such as differential settlement sliding and uncontrolled seepage, unusual vertical or horizontal movement or cracking of embankments or abutments, tilting or sliding of intake towers etc. Therefore dam deformation monitoring is imperative. It is defined as the systematic measurement and tracking of the alteration in the shape or dimensions of an object as a result of the application of stress to it. Deformation monitoring is a major component of logging measured values that may be used to for further computation, deformation analysis, predictive maintenance and alarming. A survey scheme to document the monitoring plan and its intended performance is necessary for each monitored structure. A deformation Survey data flow fig.2 adopted from US Engineering Manual No. EM 1110-02-1009 of 01Jun 02 outlined the measurement scheme and its operative procedures. Fig 2 Deformation Survey Data Flow (as adopted and modified from EM 1110-02-1009 of 01Jun 02) 6.0 APPLICATION OF GEODETIC CONTROL

The main purpose of for monitoring and analysis of dam structural deformation is to check the behavior of the dam and its environment in respect of consistency or otherwise of the predicted pattern for an early stage detection of abnormal behavior and to describe as accurately as possible the actual deformation status that could be used for the determination of factors responsible for triggering the deformation. Coordinates differencing and observation differencing are the two principal methods used to determined the structural displacement from survey data herein lies the significance of Geodetic controls usage. The latter is used for short term monitoring while the former is recommended for long term periodic monitoring. In coordinate differencing method monitoring of point position from two independent surveys are required to determine displacements by coordinate differencing. The coordinates from the two surveys are arithmetically differentiated to determined point of displacement. In Observation differencing changes in measurement made at two different time epochs are tracked. Measurements are compared to reveal any observed change. The principal parameters of consideration as contain in the US Engineering Manual no EM 110-2- 1009 of 01 Jun 2002 are; a) The absolute displacement arising from the behavior of the dam its foundation and abutment with respect to a stable control points established by external reference network (geodetic Network) .this comprises the both the Horizontal as well as the vertical displacement. The vertical displacement is measured in relation to stable bench marks. b) Relative displacement arising from the behavior of the dam, its foundation and abutment with respect to other points on the structure or even same structural element. This includes deflection and extension. Foundation subsidence and tilt are best measured with geodetic leveling. Other methods include hydrostatic leveling and Tilt meters which are permanently installed in galleries. Having multiple control stations in the reference network is critical for improving the reliability of deformation surveys and for investigating the stability of the reference monuments overtime. Therefore monitoring Scheme should include survey stations at points where maximum deformation have predicted. In any case the spatial distribution of survey monument should provide complete coverage of the structure extending to stable areas of the dam. Fig 3 and fig 4 below show some location of control points in a monitoring scheme for earth/rock fill dam and in the case of hydropower dam respectively. The recommended standard number are for the horizontal control minimum of four (4) but preferably six (6), for vertical control at least three (3) or four (4) bench marks. 7.0 DATA ANALYSIS The data obtained during monitoring expeditions be could subjected to an analysis. The Spatial displacements analysis involves using Geometric modeling. In this case general movement trends are described using a sufficient number of discrete point displacements. As an illustration Let (dn) represent discrete point displacements n Then dn (∆x, ∆y, ∆z); for n = point number In differentiating method the point displacements are calculated by differencing the adjusted coordinates for the most recent survey campaign (f), from the coordinates obtained at some reference time (i), for example: ∆x = xf - xi is the x coordinate displacement ∆y = yf - yi is the y coordinate displacement ∆z = zf - zi is the z coordinate displacement ∆t = tf - ti is the time difference between surveys. Each movement vector has magnitude and direction expressed as point displacement coordinate differences. Collectively, these vectors describe the displacement field over a given time interval. Displacements that exceed the amount of movement expected under normal operating conditions will indicate possible abnormal behavior. Comparison of the magnitude of the calculated displacement and its associated survey accuracy indicates whether the reported movement is more likely due to survey error: |dn|< (en) where 2 2 2 dn| =√ (∆x + ∆y + ∆z ) for point n, is the magnitude of the displacement.

(en) = max dimension of combined 95% confidence ellipse for point 2 2 n = (1.96) √ (σf + σi ), and σf is the standard error in position for the (final) or most recent survey, σi is the standard error in position for the (initial) or reference survey. Accuracy should be less than one-third of the predicted value for the maximum expected displacement (D max) over the given span of time between two surveys. This ensures that the total uncertainty in coordinates (plus and minus) is less than two-thirds of the total predicted movement as a minimum safety factor. P error < (1/3) D max Where P error = allowed positioning error D max = maximum expected displacement (Displacements are calculated by differencing coordinates obtained from two monitoring surveys. Therefore, the total allowable displacement error (σd) must combine uncertainty in both the initial σ1) and final (σ1) surveys: 2 2 σd = √ (σ1 + σ2 ) Where σ1 = positioning uncertainty of initial survey σ2 = positioning uncertainty of final survey Positioning accuracy will be approximately equal (σ0) if the same methods and instruments are used on each survey: 2 2 σ0 2 = σ1 = σ2 and σd =√ (2) · (σ0)

8.0 The Nigerian Situation Nigeria has witnessed an upsurge in dam construction in the past three decades. Over 323 dams have been constructed in the Country and many more are under construction in different parts of the country. It is in record that between 1970 and 1995, 246 dams were constructed in the Nigeria and that there have been several cases of dam-related disasters in Nigeria displacing thousands of people and plunging them into poverty and destroying properties. Etiosa Uyigue (2006), Some these disasters in press are as follows : (NEW AFRICAN JANUARY 1999)[1]: Living close to a dam can be a double- edged sword, as the farming communities clustered around on the Niger river, near Lokoja have found out to their chagrin. When engineers opened the

Fig3 Monitoring Scheme for earth fill/rock fill dam

Fig4 Monitoring Scheme for Hydroelectric dam

i. floodgates of Nigeria's Kainji dam without thinking of the consequences, they sent a torrent of water downstream which swept away 60 villages and the homes of hundreds of families. Surprisingly no one had thought of the consequences of letting a torrent of water gush downstream in an area already prone to flood….. On 7 October, following heavy rainfall, there was a build-up of excessive upstream water from the neighboring Niger Republic that was threatening to overwhelm the dam. The dam authorities decided the only way of reducing the pressure on the dam was to open its gates to shed the excess water. A torrent of water gushed forth bursting the river banks. The ensuing deluge swamped virtually all nearby villages. The flood tore down everything in its path and submerged about 60 villages, washing away scores of age-old mud houses ii. (This Day, September 16, 2003)[2]:- Saturday 11th September, 2003,Shiroro Dam : Over 26 villages in Kede, Lakpma and Shiroro Local Government in were flooded by the waters from Rivers Niger and Kaduna in 2003. The flood displaced about 10,000 persons in Ketsho in Kede Local Government who were said to have moved to Kwara State, while other 13,500 person in Lakpam and Shiroro were rendered homeless. In the affected areas, houses, property, farm produce and animals were destroyed by the flood which struck in the early hours of. The flood resulted from a downpour and the release of excess water from the Shiroro Hydro-Electric Dam by the National Electric Power Authority (NEPA) iii. (Daily Champion October 23, 2003) [2]:- July 2003 Obudu Dam : The Obudu Dam spillway was damaged by storm in July 2003 which resulted in fatal disaster that claimed over 200 houses, several farmlands, settlements and business concerns. The disaster was allegedly caused by the release of excess water from the Lagdo Dam in Cameroun, which overflowed Benue and Niger River bank. iv. 24th Sept 2010 LAGOS, Nigeria (AP) [2]:- Nigerian authorities opened the gates at two swollen dams in the country's rain-soaked north, sending a flood into a neighboring state that has displaced 2 million people, officials said Friday. Water from the Challawa and Tiga dams has swept through rural Jigawa state, bordering the nation of Niger… the rising waters has affected about 5,000 villages in the typically arid region approaching the Sahara Desert. "They released water indiscriminately," It wasn't immediately clear whether residents received a warning or if anyone was injured or went missing in the flooding. Officials typically open dams seasonally in the region, but it appears far more water flowed out than residents expected. v. "The flood has washed away all the farms and houses," Nigeria, Africa's most populous nation, last saw serious flooding in 2007, when 68 people died and 50,000 were affected. vi. Sunday, 1 October 2006 news.bbc.co.uk [2]:- Hundreds of people in northern Nigeria have been made homeless after a dam burst in , the state's governor has said. Governor Ahmed Sani Yarima told the BBC torrential rain had brought the water level behind the dam to critical level, forcing it to burst. Mr. Yarima said about 1,000 families had lost their homes but said that contrary to media reports no-one was killed. Earlier reports in local media said at least 40 people had died. The governor described how a wall of water swept through villages below the dam, close to the state capital Gusau. "The body of water was just like the pictures of tsunami that we've seen," he said. "It had enough force and speed to sweep people off their feet and into the river." vii. (AFP) – Sep 26, 2010[2]: -In in Nigeria's northwestern corner, local chiefs told workers from aid organizations that around 40 people were killed when a dam burst earlier this month, Nigerian President Goodluck Jonathan, who is running in elections set for early next year, toured Sokoto's flooded areas last week and pledged help. Etc. Therefore Geodetic monitoring aspects of dam structures in Nigeria can best described as completely underutilized. In its annual Dam monitoring report of the year 2004 National Electric Power Authority NEPA (Now PHCN ) has reported on pp 112 about Monitoring by survey procedures that “ We are almost neglecting this essential aspect of dam monitoring, the geodetic monitoring of the dam to measure the embankment total movement relative to undisturbed points downstream needs to be reactivated. The reference pillars need protection from vandals and disturbance by trees/shrubs growing close to them. The provision for the deflection and deformation surveys by the authority’s surveyors needs to be intensified in order to keep an old structure in checks” Kainji, Shiroro and Jebba are hydroelectric dams that are fundamental to the power generation in the Nigeria being manned by PHCN. Sadly the story of negation of geodetic survey aspect of monitoring is the same today as was report in year 2004. Though the routine Geotechnical monitoring are being conducted regularly, the historic long term records of such data were not well preserved; there was no modernization of techniques of recent data collection and archiving procedures. Recent investigations conducted in respect of some major dams the Northern States of Nigeria confirm the negation of geodetic monitoring of dam structures in the area. The investigation involved site visitation as shown on Plate 1 and in some case telephone conversations with some staff of the responsible managing authority of the dam as well as the respective State’s Ministries for Water Resources. Relevant Professionals in both the private and academic sectors were interviewed. For the Zobe, Jibia Ajiwa, Mairuwa and Sabke Dams in there are no monitoring by Survey procedures being conducted at all. Even the geotechnical monitoring is seldom done. Past records were still in analogue format are being threaten by Termites destructions. In fact apart of Zobe and Jibia dams no control points were found established. A 2004 safety review by Enplan Group (Sept 2004} reported in respect of that although the dam appeared stable, it had experienced seepage problems in the past, should be monitored closely, and should be modified to intercept the foundation seepage” The same situation exists in respect Bakolori, in Sokoto State. For Tiga and Challawa dams of the story is the same. Kano state has more than 22 operational dams in Challawa and Kano basin however no adequate facilities are in place for deformation monitoring. In Bauchi and Kaduna States there was no Standard monitoring scheme at for all the dams in the respective states. 9.0 FINDINGS Some of the findings of this work include; i. There were several instances of dam-related disasters in Nigeria resulting into lost of lives and properties over the last few decades in most river areas and mostly in the Northern part of the country. ii. There is absence of geodetic control points established for monitoring purposes in most of the dams in the Northern States and in the few dams, the existing control points are inadequate to provide the required checks. In some cases the points suffer maintenance neglect and are often vandalized or rendered unusable due inter visibility problems. iii. There is a lack of : • clear National dam monitoring policy, • adequate funding, • good instrumentation and • Competent of personnel to effectively manned monitoring schemes. iv. There is a need for the establishment of an early warning system and plans to protect lives and properties of people especially at the downstream These problems may be attributed to the absence of an articulated monitoring scheme in all the dams investigated. Thus there is an urgent for refreshers courses on dam safety and monitoring for the staff of respective dams responsible for operation and maintenance of the dam to create awareness. It also being advocated that modern Geoinformatics techniques that include the use of Global Positioning System (GPS) be employed As a supplement to existing geotechnical instrumentation, the Global Positioning System (GPS) offers a reliable and efficient method for three-dimensional monitoring. This is due to its ease of use, and capability of very high accuracy when the appropriate hardware, software and field procedures are implemented. There is apparent communication gab between the policy makers and the field professional. Therefore the challenge is for the Geoinformatics community to further create avenues of enlightenment to empower the staff and also encourage the modernization of the current instrumentation and techniques.

Plate 1.Surveyors ( the author arrowed) conducting measurements on Zobe Dam Crest in 2010 10.0 CONCLUSION AND RECOMMENDATIONS The paper has reviewed the importance of dam monitoring scheme and the role of geodetic control as means of checking the behavior of the dam and its environment. A dam with a record of safe performance may still experience failure due to undetected deficiencies in the dam or in the foundation hence the significance of dam monitoring scheme. This monitoring concern the consistency or otherwise of the predicted pattern for an early stage detection of abnormal behavior and also to aid describe as accurately as possible the actual deformation status that could be used for the determination of factors responsible for triggering the deformation. It has been show that monitoring by survey procedures has over the years suffered neglect and as result there have been several cases of dam-related disasters in Nigeria displacing thousands of people and plunging them into poverty and destroying properties. Although it is impossible to quantify the overall safety of a dam, maximum dam safety could be achieved by applying utmost care and competence to every aspect of design, construction, operation, and maintenance. Understanding some basic principles of data acquisition and signal processing is must for anyone who embarks on a program of long term monitoring of a dam or any other structure, we therefore recommend strategic manpower development through adequate geospatial education. As the scope of the monitoring methods employed depends on the potential risk associated with dam and site characteristics, newly available mapping, sensing, tracing, and visualization technologies services provided by Geoinformatics community will enable detection and monitoring changes in dam structures and indeed the in inland water ecosystems with unprecedented scope and resolution. In dam related issues Dam safety must take precedence over all other considerations, It is therefore recommended that Government should develop a National dam monitoring policy that would encourages the use of Geodetic and geotechnical inspection of dams using Geoinformatics to forestall dam related disasters. The roles of the Centre for Geodesy and Geodynamics Toro, the Federal Ministry of Water Resources and other stakeholders in this respect is vital. REFERENCES Bannister, A and S Raymond (1979) Surveying 4th edition Pitman Publishing Limited London Barry MYERS, P.E OPTIMIZATION OF DAM MONITORING SYSTEMS: REVIEW OF THE AVAILABLE TECHNOLOGY AND CASE STUDIES*Presented at the ICOLD 20thCongress – Beijing, China Barry MYERS, P.E and Jay N Statelier (2010 ) INSTRUMENTATION AND DAM MONITORING TECHNOLOGY ENCOURAGES INCREASING USE. Government Engineering ¦ September-October 2010 Www.Govengr.Com Bomford, G. (1980) Geodesy Oxford University Press. Engineering Manual EM1110-2-1009 U.S. Army Corps of Engineers (2002). Dam Monitoring. http://engineeringsurveyor.com/dam_monitoring/index.htm Enplan Group (September 2004). "Review of The Public Sector Irrigation in Nigeria". Federal Ministry of Water Resources / UN Food & Agricultural Organization. Heiskanen, W, A. and H. Moritz (1967) Physical Geodesy .W.H.Freeman, San Francisco Fernando Cesar Dias RIBEIRO et el (2008): Comparison Between Geodetic Technology And Plumb lines In Monitoring Of Displacements On Itaipu Dam, in the proceedings of the 13th FIG/4th IAG Symposium on Deformation Measurement and analysis / Symposium on Geodesy for Geotechnical and Structural Engineering Lisbon. Portugal May 12-15, 2008 http://140.194.76.129/publications/eng-manuals/em1110-2-1009/c-2.pdf ftp://ftp.fao.org/AGL/AGLW/ROPISIN/ROPISINreport.pdf. Retrieved 2010-05-21. http://www.google.com/hostednews/afp/article/ALeqM5g2EECoOQ7m50STMseDztvvCtol2A http://www.foxnews.com/world/2010/09/24/opened-flood-gates-dams-northern-nigeria-displace- million-people/#ixzz1ULqlBP69 http://www.fig.net/commission6/lisbon_2008/papers/pas05/pas05_03_diasribeiro_mc061.pdf http://news.bbc.co.uk/2/hi/africa/5396176.stm http://en.wikipedia.org/wiki/Deformation_monitoring