Introduction to Control Surveying

Control Surveying Control Surveying Control Surveying is the determination of the precise position of a series of stations distributed over an area to serve as the origin or reference to be used for checking of subsequent surveys to be used in engineering projects like property delineation, topographic and hydrographic mapping, and construction planning and design. Control networks which cover the whole country have become been conducted with better accuracy and less stringent technique, with the use of artificial satellites. These stations are linked to local networks which have been adopted for special surveys connected with projects such as dams, roads, railways and pipelines, large or small construction sites, etc. The purpose of a control system is to prevent the accumulation of errors, by connecting detail work to a consistent geometrical system of points, which are accurate enough for the project. Great care is taken to ensure that this control is sufficiently accurate. There was a time when geodetic control points consisted of triangulation networks marked by observation pillars. In the Philippines, many triangulation stations have been located on top of towers which had been placed on mountain summits to answer the problem of intervisibility. But because of the ease with which positions can be established by satellite systems, which eliminated the intervisibility requirement, there has been less need for establishing so many points. Gradually, as the scope of the survey becomes smaller, the use of non-satellite systems to provide control becomes more prominent. Control underground, in urban streets and inside buildings is predominantly carried out by terrestrial methods. Apart from their use with further ground survey operations, control points are also required to augment photogrammetric and remote sensing methods of mapping. Plan coordinates and heights of points identifiable on imagery are needed by all but the most sophisticated systems. Classes of Control Surveys There are two classes of control surveys namely: (1) geodetic control survey; and (2) project control survey. The classification of the control survey is essentially based on the accuracy of which the distances and directions of the lines inside their traverse, triangulation, trilateration or GPS networks have been determined. Geodetic control surveys consist of triangulation, traverse, trilateration, GPS, and any combination, which together with leveling and astronomic observations, determine the accurate geographic positions of points on the earth’s surface taking into account the curvature of the earth. It also includes the determination of position of points through satellite survey or photogrammetric aerial triangulation. Geodetic control surveys shall be made in accordance with the general instruction contained in the special publications used by the Coast and Geodetic Surveys Department of NAMRIA for the first, second, and third order control work. First Order Geodetic Control Second Order Geodetic Control Third Order Geodetic Control Project control surveys consist of traverse, triangulation or any combination, which together with leveling, shall determine the positions of control points between stations of geodetic accuracy over an area of limited extent, such as isolated tracts of lands, group settlements, barangay areas, municipalities or group of municipalities. There are three classes of project control namely: Primary Control – The azimuth of primary control shall be determined to the nearest one second of the arc, and the distances shall be measured twice to the nearest millimeter, taking into account the temperature, sag, pull, grade and sea level correction when using a survey tape. Secondary Control – The azimuth of secondary control shall be determined to the nearest fifteen (15) seconds of arc or less and the distances shall be measured once to the nearest millimeter, taking into account the temperature, sag, pull, grade and sea level correction when using a survey tape. Tertiary Control – The azimuth of tertiary control shall be determined to the nearest one minute of arc or less and the distances shall be measured once to the nearest centimeter, taking into account the temperature, pull, sea level and sag corrections when using a survey tape. Control Stations Control stations are usually small marks set immovably into the ground such that the instrument or optical target can be set up above it, to an accuracy of about 1mm in the horizontal plane. These would include the horizontal control points (latitude/longitude or plane coordinates), vertical control points (orthometric or ellipsoidal heights), gravity values, and in some but rare cases, crustal motion values (movement of points). A control point may have one or more of these values attached to it. In the past, benchmarks (vertical control points) and horizontal control points are separately and independently established, but today’s common practice in creating control networks demands establishment of points with known horizontal and vertical measurements. Control points in the survey must be consistent with all other points in the geodetic control and not just within that particular survey. Location of Control Stations Control stations are not usually positioned in an exactly predetermined position. The normal process is to choose a location where a control station would be useful and is secured from being disturbed or moved. After putting up the station, precise measurements are then taken to determine exactly where it has been placed. The factors which influence the positioning of a control station are as follows: If it is to be used for setting out, or for determination monitoring, then it should be placed where all relevant places and features can be easily seen, without the line of sight passing close to another object such as a building or hillside. If the station is to be used in a conjunction with other similar stations for these purposes, then the different lines of sight from the stations should form a well-conditioned shape, so that the positions of the observed points will be found to be the greatest possible accuracy. If the exact position of a new control station is to be fixed by conventional means then it must be visible from at least two other control stations (and preferably from more). Sometimes, additional control stations are introduced into a network simply because they will be visible to several ‘useful’ stations and will therefore improve the accuracy to which the positions of those stations are known. If the station is to be used for GPS, then a large area of sky should be visible at the station (particularly towards the equator), and there should not be any high walls nearby which might reflect satellite signals towards the receiver. If an instrument is to be left unattended at a station, then the station must be in a secure place such that the instrument cannot be stolen or disturbed while the surveyor is elsewhere. As far as possible, a station should be sited in a place where it will be easy and safe to use, far away from noise, vibration or traffic, and unlikely to be disturbed or destroyed during its anticipated useful life. Stations sited near roads or on tarmac pathways are always at risk of being covered over, and lost without trace. Stations in the middle of building sites are at risk of being dug up, or run by heavy construction traffic. The latter may not be destroy a station, but it could move it slightly – and thus cause all subsequent observations involving the station to be subtly inconsistent with those made beforehand. Appearance of Control Stations The type and/or appearance of marker used vary with the following factors: Type of soil or material at the marker site Degree of permanence required Cost of replacement Precision requirements In open ground, a short-term control station might be a 1mm diameter hole or ‘center pop’ in a brass tack driven into a short (30cm) wooden stake, which is then hammered into the ground. On tarmac, it might be a center mark on a stainless steel ‘road bolt’ which is likewise driven into the ground. Such road bolts normally have a hemispherical head with a diameter of about 5mm, on top of a fixed disk about 20mm in diameter. They may also have a plastic washer, or a circle painted round them, designed for identification purposes. For a more permanent marker in open ground, a pre-cast toughened concrete block with a suitable marker on its surface might be dug into the ground, so that only its top surface is visible. Alternately, a hole can be dug with some ferrous reinforcing bars arranged inside it and a quantity of concrete poured in with a non-rusting marker fixed so as to emerge slightly above the surface of the concrete when it has set; a small solid brass doorknob, some threaded steel rod which it will screw onto and some ready mix concrete for fence posts is all that is require d. This gives an extremely durable station at very modest cost, which has the added advantage that it can be covered over with a piece of turf or layer of soil, and thus escape the risk of being vandalized when not in use. If the upper surface of the marker is spherical, then its highest point can also conveniently be taken to be the height of the station. A control point on a construction site would normally be surrounded by a small rectangular ‘fence’, made of brightly painted wood, to warn drivers of its existence. This reduces the likelihood of the station being run over by a heavy vehicle, and a broken fence gives a helpful indication that this may have occurred. Control Survey Control Surveys are used to establish precise horizontal and vertical positions of reference monuments, which serve as the basis for originating or checking subordinate survey projects. Control stations are vital in the establishment of the country’s national geodetic framework, and are also essential as references for giving locations of data entered into Land Information System (LIS), and Geographic Information System (GIS). Field Organization The organization of fieldwork forms a very important part of the surveyor’s work. It entails the preparation of an efficient technical program, which is both cost effective and acceptable to the workforce. Because surveying practice is still weather dependent the work cannot be scheduled to normal working hours. It is also affected by the site environment. Surveying near an operating railway or busy motorway is particularly hazardous: work in tropical forests is a danger to health; sub-zero temperatures call for special clothing; and transport demands the proper care and maintenance of vehicles or boats. The provision of food, accommodation and fuel has to be attended to, not to mention the recruitment and payment of temporary staff and care for their health. Technical operations can often be the easiest part of any task. Technical Procedures The surveyor must be fully conversant with his instrument if the best results are to be obtained. The centering of instruments and targets over reference marks needs continual vigilance, and all tripods and plummets need to be secure and adjusted. For the best results on control work, horizontal and vertical angles should be taken separately, the former on various arcs or zeros. Independent pointings are necessary to remove systematic bias. A regular field booking procedure should be adopted to avoid omission of vital dimensions, such as heights of instruments and targets. In the case of EDM, proper pointing procedures and meteorological measurements are essential, as is the attention to batteries and their charging. A technical program must have sufficient flexibility to respond to weather and other unpredictable factors. Radio communication adds greatly to efficiency and general security. Planning a Control Scheme All control schemes require careful planning to suit the task in hand and any likely future demands. Various survey methods are being used: GPS fixes to give differential vectors between stations; an astronomical station to give information about the geoid and azimuth control; EDM lines forming part of a lateration scheme (trilateration or triangulation); and some radiations to fix photogrammetric control points in plan and in height. Other methods might be used such as intersection, resection, and traversing, although not on the same kind of task. In past times the surveyor was somewhat inhibited in his choice of method to fix controls because of the tedium of computation. This situation has all but disappeared in most countries. Of particular interest in this respect is resection, now much employed throughout surveying because of the ease with which computer processing has replaced the thirty-minute calculation by hand machine. Processes of Control Survey The process involved in carrying out the survey can be itemized as follows: (1) Doing Reconnaissance: Reconnaissance is the examination of a project area to assess the overall feasibility of the fieldwork portion of a project. The following are some of the many goals to be determined in the preliminary survey: To assess accessibility of the project area and possibility of alternative project routes To assess the conditions of existing project control and possibility of alternative project points To assess feasibility of field methods and alternative techniques To assess limitations in the environmental conditions (2) Laying the Stations: The following are some of the many goals to be determined in the laying of the station markers: To assess the appropriate type of survey markers to use To assess the intervisibility of desired project points To assess the sky visibility at desired project points in case astronomic observations or GNSS will be used in positioning (3) Distance Measurement: Various techniques are used in measuring lengths and distances, from taping to EDM’s, and more recently through satellite technology techniques. (read chapter on distance measurement) (4) Direction Establishment: Bearing and angle measurements are essential measurements in surveying. From using astro-geodetic methods of positioning to using Greco-Roman instruments, establishing direction has been employed for various survey operations (read chapter on direction establishment) (5) Position Determination: Determination of coordinates [plane(x, y, z) or map(E, N, elevation) or geographic(latitude, longitude, height)] Horizontal Control Horizontal control is provided by two or more points on the ground, permanently or semi-permanently monumented, and precisely fixed in position horizontally by distance and direction, or coordinates. Horizontal control can be established by the traditional ground surveying methods of precise traversing, triangulation, trilateration, and a combination of these basic approaches, or by the more modern methods like GPS. In addition, astronomical observations have been used to determine azimuth, latitude and longitude. Rigorous photogrammetric techniques have also been used to densify the control in an area. Until recently, triangulation and trilateration were the most economical procedures available for establishing basic control for mapping projects extending over large areas like for regions and states. These techniques have now given way to GPS, which has not only proven to be highly accurate, but also more efficient. Monuments whose positions have been established through higher-order control surveys and referenced in the state plane coordinate systems, are used to initiate surveys for all types, but unfortunately more are needed in most areas. Vertical Control Vertical control is provided by benchmarks in or near the track to be surveyed, and it becomes the foundation for correctly portraying relief on topographic maps. Vertical control is usually established by running lines of differential levels starting from and closing on established benchmarks. Project or temporary benchmarks are established in strategic locations, normally located near and around the project area, and their elevations are determined by including them as turning points in differential leveling lines. With the rapidly growing popularity of total stations, trigonometric leveling has become practical, and is now frequently used to establish vertical control for mapping, especially in rugged areas. GPS surveying may also be suitable for establishing vertical control but the ellipsoidal heights determined must first be converted to orthometric height to become useful. Accuracy Standards and Specifications Accuracy standards and specifications are required accuracy for a control survey depends primarily on the purpose. These are normally established or created by a government agency related to surveying and/or mapping. The major factors that affect accuracy include the following: Type and Condition of Accuracy Field Procedure Capabilities of Survey Personnel Importance of Standards Accuracy standards are used to provide a uniform set of standards specifying minimum acceptable accuracies of control survey for various purposes. These would also establish specifications for instruments, field procedures, and misclosure checks to ensure that the intended level of accuracy is achieved. Reference Datum Horizontal and vertical datums consist of a network of control monuments and benchmarks whose horizontal positions and/or elevations have been determined by precise geodetic control surveys. These monuments serve as reference points for originating subordinate surveys of all types. Philippine Horizontal Datum In the Philippines, the national horizontal datum is the Luzon Datum . The Luzon datum was established using two reference points in the province of Marinduque, which is considered as the geometric central island of the country. These two points, namely the Balanacan and Baltazar control points, comprise the initial baseline for the entire triangulation network with the Balanacan point in Marinduque as the starting point with known geographic coordinates. With a known azimuth and distance of 38 km away from Balanacan, the geographic coordinate of Baltazar had been calculated. A series of polygons were then established to create the triangulation network and establish the triangulation stations all over the Philippines. Hydrographic surveys were then tied to the triangulation stations. Philippine Vertical Datum The vertical datum was established by continuous observations of the fluctuations of tides during a period of about 18 to 19 years, which is equal to one complete tidal cycle. The greatest elevation of water achieved for one day is termed as HIGH TIDE. On the other hand, the greatest depression of waters in a given day is referred to as LOW TIDE. The HIGH WATER is the maximum height to which the water surface rises above the standard datum plane during a given period. When several observations of high water are averaged, it is referred to as the MEAN HIGH WATER, the mean of all high waters. For ground elevation measurements, the mean sea level (MSL), which is the average elevation of the water at all tidal stages, is used. For hydrographic depth measurements, the Mean Lower Low Waters (MLLW), which is the cyclical average elevation of the lower of the two low tides, is employed.