Introduction of Digital Topographic Survey
For Topographic Survey we can use two senses of this earth’s surface. One is Geo synthesis & another one is Geology. Geo synthesis is a combination of nature and the surface structure of the earth. In which the functions artificially created on the earth’s surface.
Geology refers to the combination of the natural and external structures of the earth’s surface.
A digital topographic survey is the position of different points on the earth’s surface and the external structure. And surveyors use advanced equipment to quantify them. The feature of a topographic survey is determining the location of different contour lines and depicts them on the map. The vertical coordinates of certain points are determined with respect to a certain point.
Geospatial map features
The feature of a geospatial map is to depict the three-dimensional position of different types of objects in an area of the earth’s surface.
Reference Objects for Topographic Survey:
- Relief including hills and valleys
- Natural features, such as streams, rivers, lakes, forests, trees, etc.
- Artificially constructed properties, such as roads, railways, canals, ponds, houses, buildings, fences, agriculture, etc.
Uses of Geo-Maps
The use of geostationary maps is very effective in designing and selecting sites for any engineering project such as
- Railways, Highways, Irrigation, Water Supply, Sewerage, Reservoirs, etc therefore essential in all these cases.
- This map is very important for Geologists and Industrialists.
- To military commanders, especially during wartime
- This map is very helpful for environmental research and development
In Bangladesh, this map is prepared by the Survey of Bangladesh.
The approved scale of the geospatial map:
In geo-geological surveys, the limits of the scale recommended for the preparation of maps are- 1 cm ratio 2.5 km (RF = 1 / 250,000) From 1 cm ratio 0.25 km (RF = 1/25000).
How is Topographic Survey Done?
Land surveyors do a conventional survey on the surface of the land with an instrument which is the Total Station. Some surveyors today may also use a survey-grade GPS unit to do this work. But that is manual work. A surveyor walks the entire site and keeps taking shots (horizontal location and elevation) at a specific interval in order to meet the project’s accuracy requirements.
Survey Process Includes those:
- A 1-foot contour topographic survey might require shots at a 20-30 foot interval. And also take shots along all break lines. A break line is a point where the contour changes abruptly. For instance the back of a curve or the centerline of a roadway.
- An aerial topographic survey is done from an airplane or helicopter at a certain distance above the ground. Consequently, some aerial photos are taken from there. The aerial photos are then checked out together to form a stereo pair. In conclusion that allows the mapper to “visualize” the surface of the ground. Two overlapping aerial photos to see the ground surface is exactly the way we have depth perception using both our eyes.
- The same point is seen from two different vantage points. From this, our brain can perceive the relative distance from our eyes. With only one photo or one open eye, you cannot perceive depth.
- A conventional survey generally occurs when the contour interval and spot elevations must be exact.
- The aerial mapping process is more limited in accuracy on both the horizontal and vertical locations of points on the surface.
- A 1-foot contour is about the limit for aerial work and spot elevations may be accurate to only a half-foot. So, this type of survey is not acceptable for designing something like a sanitary sewer. And that might need to be within ±0.02
While these are somewhat simple explanations of the approaches for doing a topographic survey. The purpose of this explanation is only to give an overview. What does a completed Topographic Survey look like?
The output of a Topo Survey
The final product of a topographic survey is map. That map constructs with all of the required features along with the contours representing elevations. An electronic map may also be supplied to an Engineer or Architect for their use in further design. The electronic file will have all the spot shots and breaklines. And that file also includes a digital terrain model (DTM). All of these breaklines and spot shots might not be represented on the printed version of the map.
Field Work for Topographic Survey
Surveying helps determine accurately the terrestrial or three-dimensional space position. Consequently Surveying is an aggregation of points and distances and angles among them. And Certainly, surveyors use leveling instruments such as Theodolites, Dumpy Levels, and Clinometers.
A Brief History of Topographical Survey
Giovanni Domenico Cassini was the first topographic maps developer. He was an Italian astronomer. Most importantly remote sensing has greatly sped up for that map. And then survey science got greater accuracy control over long distances. Direct survey still provides the basic control points and framework. Furthermore direct survey workers on manually or GIS-based. Most of Europe and the Continental U.S has extensive direct survey and mapping program. The data from that survey forms the basic digital elevation datasets for instance “USGS DEM data”. This data must often be “cleaned”. Otherwise, the elimination of discrepancies between surveys seems difficult. But it still forms a valuable set of information for large-scale analysis.
The original American topographic surveys (or the British “Ordinance” surveys) involved not only recording of relief, but identification of landmark features and vegetative land cover.