The topographic survey involves determining the horizontal and vertical locations of objects on the surface of the earth. Horizontal location entails locating objects like roads, railways, ponds, houses, boundaries of properties etc. by measuring horizontal distances the objects are indicated by symbols.
Vertical location includes the location of hills, valleys, depressions, benchmarks, etc. by measuring vertical distances the objects, in this case are represented in relief.
· Thus, a topographic map shows the nature of the earth surface along with the positions of different objects.
· Such a map is essential for the engineering projects involving roads, railways, irrigation, reservoirs, townships etc.
· The scale of a topographic map depends on the extent of area it covers, and the purpose for which it is to be prepared.
· Generally topographic map is prepared according to a scale of 1 cm to 1 km (i.e. 1/100,000).
Representation of Relief
From the general map of a country, only the positions of objects can be found out, angles of the traverse. The lengths of traverse legs are measured accurately by using the total station if the area is extensive; it is divided into a number of sectors. Each sector is enclosed by a closed traverse having a proper connection with the other sectors (it is ensured sectors have that the common sides).
Location of objects
In the case of a triangulation survey, the objects are located by taking offsets with respect to the survey lines. The offsets are noted in a field book, maintaining a proper sequence of survey lines (AB, BC, CD). Then a map is prepared by plotting field records according to a suitable scale.
In the case of total station traversing, first, the traverse is plotted to a suitable scale by the coordinate method (on the basis of latitude and departure) and then the objects are located on the map by plane table by radial and intersection methods or by the total station. If the objects are located by the transit-and-tape method, a field book has to be entered while measuring the traverse legs. In this case, the plotting is done afterward.
The term route survey refers to surveys necessary for the location and construction of lines of transportation or communication that continue across the country for some distance, such as highways, railroads, open-conduit systems, pipelines, and power lines. Generally, the preliminary survey for this work takes the form of a topographic survey. In the final stage, the work may consist of the following.
· Locating the center line, usually marked by stakes at 100-ft intervals called stations.
· Determining elevations along and across the centerline for plotting profile and cross-sections.
· Computing the volumes of earth work and preparing a mass diagram
· Locating right-of-way boundaries, as well as staking out fence lines, if necessary.
As mentioned earlier in this chapter, Special Surveys are conducted for a specific purpose and with a special type of surveying equipment and methods.
Land Surveys (sometimes called cadastral or property surveys) are conducted to establish the exact location, boundaries, or subdivision of a tract of land in any specified area. This type of survey requires professional registration in all states. Presently, land surveys generally consist of the following chores:
· Establishing markers or monuments to define and thereby preserve the boundaries of land belonging to a private concern, a corporation, or the government.
· Rerunning old land survey lines to deter- mine their lengths and directions. As a result of the high cost of land, old lines are remeasured to get more precise measurements.
· Calculating areas, distances, and directions and preparing the land map to portray the survey data so that it can be used as a permanent record.
· Writing a technical description for deeds.
The line of intersection of a level surface with the ground surface is known as the contour line or simply the contour. It can also be defined as a line passing through points of equal reduced levels.
For example, a contour of 100 m indicates that all the points on this line have an RL of 100 m. Similarly, in a contour of 99 m, all points have an RL of 99 m, A map showing only the contour lines of an area is called a contour map.
The horizontal distance between any two consecutive contours is known as horizontal equivalent. It is not constant. It varies according to the steepness of the ground. For steep slopes, the contour lines run close together, and for flatter slope’s they are widely.
Object of Preparing Contour Map
The general map of a country includes the locations of roads, railways, rivers, villages, towns, and so on. But the nature of the ground surface cannot be realized, from such a map.
However, for all engineering projects involving roads, railways, and so on, knowledge of the nature of ground surface is required for locating suitable alignments and estimating the volume of earth work.
The process of tracing contour lines on the surface of earth is called contouring. A contour map gives the idea of the altitudes of the surface features as well as their relative positions in a plan.
Uses of Contour Maps
The following is the specific uses of the contour map:
· The nature of the ground surface of a country can be understood by studying a contour map. Hence, the possible route of communication between different places can be demarcated.
· A suitable, site or an economical alignment can be selected for any engineering project.
· The capacity of a reservoir or the area of a catchment can be approximately computed.
· The indivisibility or otherwise of different points can be established.
· A suitable route for a given gradient can be marked on the map.
· A section of the ground surface can be drawn in any direction from the contour map.
The aim of leveling is to determine the relative heights of different objects on or below the surface of the earth and to determine the undulation of the ground surface.
Uses of Contour Maps
Leveling is done for the following purposes:
· To prepare a contour map for fixing sites for reservoirs, dams, barrages, etc., and to fix the alignment of roads, railways, irrigation canals, and so on.
· To determine the altitudes of different important points on a hill or to know the reduced levels of different points on or below the surface of the earth.
· To prepare a longitudinal section and cross-sections of a project (roads, railways, irrigation canals, etc) in order to determine the volume of earth work.
· To prepare a layout map for water supply, sanitary or drainage schemes.
Control Surveys provide “basic control” or horizontal and vertical positions of points to which supplementary surveys are adjusted.
These types of surveys:
· Sometimes termed geodetic surveys) are conducted to provide geographically.
· Positions and plane coordinates of triangulation and traverse stations and the elevations of benchmarks.
· These control points are further used as references for hydrographic surveys of the coastal waters, for topographic control and for the control of many state, city, and private surveys.
RAILWAY HIGHWAY & ROAD SURVEY:
A railway project should be implemented in different stages, as described below:
Justification of Constructing New Line
A When the question of constructing a new railway, Road line between some places arises due to public demand or some other reason, an intensive study is carried out as regards viability of the project:
· The following points should be kept in mind while considering the justification of constructing a new line.
· Total population of the villages, towns, industrial places, etc, coming –under the project, should be recorded.
· The standard of living and habits of the population, and the economic conditions of the locality, should be thoroughly studied as it will be a source of revenue for the department.
· The amount of agricultural, natural and industrial resources should be recorded.
· Information regarding religious places, religious fairs of festivals, business centers, etc. should be noted.
· The amount of revenue that may accrue from passengers, agricultural goods, industrial goods, natural resources, etc. should be estimated.
· A topographical map should be studied to determine the number of culverts, bridges, level crossings, tunnels, etc.
· An agricultural map should also be studied so that too much valuable land is not affected.
· A contour map should be studied in order to ensure economical alignment, avoiding unnecessary cutting or filling and maintaining a permissible gradient.
· An Industrial map should be studied to find the shortest route to connect industrial areas.
Marking of Tentative Alignment
After the decision to set up a new railway line, Road is taken, tentative alignments of alignments are marked on the contour map and topographical map for the area concerned. While marking the tentative alignment, the following points should be kept in mind:
· The route should be the shortest possible.
· As far as possible, it should follow the ridge line to avoid unnecessary earth work in filling.
· It should cross rivers perpendicularly.
· It should not pass through religious places, such as temples, mosques, churches etc. or burial grounds, burning ghats, and soon.
· The route should not pass through the centers of villages, towns, valuable structures, etc. where the compensation payable is likely to be more.
· It should not be too zigzag, so that the number of curves is minimized.
· It should not cross large depressions or valleys, high summits or ridge lines, to avoid huge earth fillings in areas of depression and tunneling in regions including summits.
· The tentative alignments of two routes should be marked for comparing costs of construction, feasibility, advantages, etc.
Selection of Good Alignment
After reconnaissance survey, a good alignment or alignments are selected for preliminary detailed survey.
Detailed Preliminary Survey
After selection of suitable alignment of alignments, a detailed preliminary survey is conducted. Such survey should be extremely accurate as selection of the final alignment depends on it.
Preparation of Estimate
· Earth work required in cutting and banking is estimated.
· A mass diagram is prepared fro balancing the cutting and banking operations.
· An approximate estimate is done for culverts, bridges, levels crossings, etc. (A final estimate is prepared after the design.)
· A compensation list is prepared showing full details.
· A detailed costing of the project is then prepared for inclusion in the project report.
Final location Survey
The alignments considered in the preliminary survey, the most economical is selected. Before the approval of the project, the final location survey is completed in all respects. This survey helps the engineers in charge of construction projects.
Project Report (DPR)
After completion of all survey work, Estimation a project report should be prepared and submitted for approval.
A project on township or city surveying involves proper coordination of all types of development work necessary for the township or city. It entails implementation of the following systems:
· Street system
· Property lines
· Water supply system
· Sanitary system
· Electrification system
· Telephone system
In the case of a newly developing township or the modification of one in existence for some time, a plan for development should be clearly worked out so that the facilities provided in the master plan may be suitably extended.
Instruments Required for Conducting City Survey
The following instruments are required for conducting a city survey:
· Total station with stand
· Leveling instrument with stand
· Leveling staff
· Metallic tape
· Invar tape
Preparation of Topographic Map
The entire area is divided into a number of sectors, each of which is enclosed by a polygon. The polygons are connected by common sides. They are treated as closed traverse and the traversing is done by total station. The interior details are located by plane table or the transit-and tape-method. These details include houses, roads, lakes, parks, railway lines, stations etc.
Fly leveling is undertaken to establish the RLs of important points and benchmarks. Contouring is done by plane table or tachometer. The nature of the ground surface is indicated in relief (coloring, shading, hachuring, etc.) in the map.
Finally, all the sectors are assembled in one map so that the township area or city area.
Preparation of Street Map
The street map is prepared using a large scale to show distinctly the network of streets, roads, lanes, parks, etc. Names of streets, roads and parks should be mentioned. In this case also the township area is divided into different sectors. The street map of each sector is prepared by plane table. In such a map the location of interior details (lakes, houses, and other properties) need not be shown. Fly leveling is done along the streets and benchmarks are established at different points for future reference.
Preparation of Property Map
This map is also prepared using a large scale to show the boundaries of public and private properties, plot numbers, premises numbers, and so on. The property map is also prepared by plane table by dividing the total township area into different sectors.
Preparation of Water Supply and Sanitary Map
The network of the water supply distribution system and that of sewer lines are laid underground. So, to facilitate location of any spot from top, the entire networks of the water supply and sanitary works are shown by conventional lines on the street survey map. All the essential lines and points (such as intake points, purification point, check valves, fire hydrants, manholes, loopholes, inspection chambers, etc,) are marked with specific.
Preparation of Electrification Map
Generally, the network of cables for electrification of the township or city is also laid underground. Therefore, the methods of distribution should be marked on the street survey map using a different color convention or by suitable symbols. The specific points of the supply line should be clearly marked to aid easy location of any spot in case of cable fault. The network of the supply system should be indicated on the street survey map even if the electrification is through overhead lines.
Preparation of Telephone Map
The network of telephone cables also lies under the ground. So, to aid location of a fault, the network system is shown on the street survey map using a certain color convention or by suitable symbols.
The development authority of the city or town is responsible for the construction of streets, roads, culverts, bridges, flyovers, bypasses, subways, parks, public places, etc. The sanitary and water supply system is implemented by the municipality or corporation. The electrification program is carried out by the state electricity board, and the telecommunication system is run by the telegraph department.
GPS & DRONE SURVEY:
GPS is an exceedingly precise route framework utilizing signals from satellites to decide an area on the Earth’s surface, independent of climate conditions.
Studying and mapping was one of the primary business adjustments of GPS, as it gives a scope and longitude position straightforwardly without the need to quantify edges and separations between focuses.
Be that as it may, it hasn’t completely supplanted reviewing field instruments, for example, the theodolite, Electronic Distance Meter, or the more current Total Station, because of the cost of the innovation and the requirement for GPS to have the capacity to ‘see’ the satellites accordingly confining its utilization close trees and tall structures.
Drone Aerial Survey
Drone Aerial Survey is a strategy for gathering geometrics or other symbolism by utilizing planes, helicopters, UAVs, inflatable’s or other flying techniques.
The utilization of unmanned aerial frameworks (UAS) is quickly growing as geospatial and development organizations figure out how much less demanding and more financially savvy finishing reviewing and mapping undertakings can be with rambles. Automatons likewise dispose of the requirement for people to physically get to hard-to-reach and unsafe landscape.