UNIT 3
Mapping & sensing
Surveying and mapping is fundamental to all civil engineering and roadway design work. It is a vital function linking the interdisciplinary elements of a project from planning through design, right of-way acquisition and construction, to final asset management. Surveying and mapping connects the real world of site conditions, terrain and improvements to the virtual world of design concepts and calculations depicted in the CADD environment. To the implementation of designs in construction, surveying and mapping reverses that connection, taking the critical elements of design from CADD and accurately placing them in the context of the real world. Land surveying and right-of-way, a specialized aspect of surveying and mapping, considers the legal interpretation of evidence related to land boundaries and real property rights, to provide a comprehensive description of the physical and legal constraints to project development.
Contours
A contour is an imaginary line on the ground that passes through points having the same elevation.
Characteristics of contour lines .
- Contour lines are continuous.
- Contour lines are relatively parallel unless one of two conditions exists.
- A series of V-shape indicates a valley and the V’s point to higher elevation.
- A series U shape indicates a ridge. The U shapes will point to lower elevation.
- Evenly spaced lines indicate an area of uniform slope.
- A series of closed contours with increasing elevation indicates a hill and a series of closed contours with decreasing elevation indicates a depression.
- Closed contours may be identified with a +, hill, or -, depression.
- Closed contours may include hachure marks. Hachures are short lines perpendicular to the contour line. They point to lower elevation.
- The distance between contour lines indicates the steepness of the slope. The greater the distance between two contours the less the slope. The opposite is also true.
- Contours are perpendicular to the maximum slope.
- A different type of line should be used for contours of major elevations. Common practice is to identify the major elevations lines, or every fifth line, with a bolder, wider, line
Profile levelling
The process of determining the elevations of a series of points at measured intervals along a line such as the center line of a proposed ditch or road or the center line of a natural feature such as a stream bed. Normally we will assign an elevation of 100.00 to the datum rather using the mean sea level elevation.
An extension of differential levelling Elevations are determined in the same manner.
This type of levelling is also called as longitudinal sectioning. In high way, canal, railway or sewage line projects profile of the ground along chooses routes are needed. Along the route, in such cases, at regular interval readings are taken and RL of different points are found. Then the section of the route is drawn to obtain the profile. Given figure shows the plan view of the scheme of levelling and fig b shows the profile of the route. Vertical scale is usually larger compared to scale for horizontal distances for drawing profile of the route. It gives apparent picture of the profile of the route.
Cross sectioning
Cross sections are lines of levels or short profiles made perpendicular to the center line of the project. For example, taking a cross section of a stream bed while doing a profile survey of the stream. Cross sections are usually taken at regular intervals and at sudden changes in the center-line profile.
The cross sections must extend a sufficient distance on each side of the center line to provide a view of the surrounding terrain.
Rod readings should be taken at equal intervals on both sides of the center line and at significant changes in the terrain.
Measurement of areas, volumes
Area computations
Area computations are commonly used to compute payment quantities for such measures as clearing, clearing and grubbing, vegetation, critical area shaping, land levelling, etc. Concrete slabs may also be paid for on an area basis. As with linear computations, it is important to know if slope distance or horizontal distance will be used to compute the area. This will generally not be specified. If payment is made on an acre basis with little relief in the topography, computing the area using either horizontal or slope dimensions would yield similar results, and either method would be acceptable unless otherwise specified. For areas where there is significant relief in the topography, the difference in the area computed using horizontal distances and that using slope distances may result in a significant quantity and price difference for the overall work. For example, slope distances would be used to compute the quantity of a pit-type pond liner because there is significant difference in the slope distance and the horizontal distance across a pit-type pond.
Volume computations
Volume computations are used to compute quantities of earth fill, excavation, concrete, rock riprap, etc. Some volumes are defined by common geometric shapes; formulas for these are given here. Other volumes are defined by irregular boundaries and must be computed by methods such as the average end-area method or grid method, which are described later in this section.
Survey stations are the points at the beginning and the end of a chain line. They may also occur at any convenient points on the chain line. Such station may be:
1. Main stations.
2. Subsidiary stations
3. Tie stations.
Main Stations
Which stations taken along the boundary of an area as controlling points are known as ‘main stations’. The lines joining the main stations are called ‘main survey lines’. The main survey lines should cover the whole area to be surveyed.
Subsidiary Stations
Which stations are on the main survey lines or any other survey lines are known as “subsidiary station”. These stations are taken to run subsidiary lines for dividing the area into triangles, for checking the accuracy of triangles and for locating interior details.
Tie Stations
Tie stations are also subsidiary stations taken on the main survey lines. Lines joining the tie stations are called tie lines. Tie lines are mainly taken to fix the directions of the adjacent sides of the chain survey map. These are also taken to form ‘chain angles’ in chain traversing, when triangulation is not possible. Sometimes tie lines are taken to locate interior details.
Remote sensing is the process of acquiring information, detecting, analyzing, monitoring the physical characteristics of an area by recording it is reflected and emitted radiation energy without having any physical contact with the object under study. This is done by capturing the reflected radiation/energy.
Types of remote sensing
1. Active sensor
The sensor embodies within itself the source of illumination like a satellite equipped with a RADAR sensor. Active sensors throw their own energy to scan the object. RADAR and LiDAR are examples of active remote sensing which measure the time delay between emission and return.
2. Passive sensor
The sensors gather radiation that is emitted or reflected by the object or surrounding areas. Sunlight reflection is the most common source of radiation measured by passive sensors. Examples of passive remote sensors are photography, infrared, and radiometers. Passive sensors are more used because it provides great quality satellite imagery. The passive sensor is superior within the field of technical observation of the planet, such as Multispectral and Hyper spectral technology.
Remote Sensing Major Applications Area
Satellites play a huge role in the development of many technologies like world mapping, GPS, City planning, etc. Remote Sensing is one of the many innovations that were possible, thanks to the satellites roaming around the earth.
Following are some major fields in what can remote sensing be used for
- Weather
- Forestry
- Agriculture
- Surface changes
- Biodiversity
The primary application of remote sensing
Analyzing the condition of rural roads
Rural road conditions are now possible to be analyzed using various GIS technique and Remote Sensing techniques with an inch to inch accuracy. It saves a lot of time and money from transporters.
Creating a base map for visual reference
Nowadays many modern mapping technologies are based on Remote Sensing including Google maps, open street maps, Bing maps, NASA’s Globe view, etc.
Computing snow pack
Snow melt ratio can be easily understood by using Remote Sensing technology, NASA uses LIDAR along with a spectrometer in order to measure the absorption of sunlight.
Collecting earth’s pictures from space
Many space organization has a collection containing images of earth. Interesting patterns of earth’s geometry including atmosphere, oceans, land, etc can be seen in it. EO-1, Terra, and Lands at are used to collect this data.
- Controlling forest fires
Information acquired by satellites using Remote Sensing enables fire fighters to be dispatched on time and over correct locations so the damage from such fires can be decreased to minimal.
- Detecting land use and land cover
Remote Sensing technologies are used to determine various physical properties of land and also what it is being used for (land use).
- Estimating forest supplies
MODIS, AVHRR, and SPOT are regularly used to measure the increment/decrement in global forests since forests are the source of valuable materials such as paper, packaging, construction materials, etc.
- Locating construction and building alteration
Tax revenue agencies use satellite data in several countries including Greece, Athens, etc. They locate signs of wealth using this technology. Early in the year of 2013, there were 15000 swimming pools (unclaimed to steal taxes) in those countries.
- Figuring out fraud insurance claims
Many insurance companies use Lands at red and infrared channels to figure out vegetation growth in particular land. This information can be used to verify seeded crops and fight against crop insurance fraud.
- Observing climate changes
Satellites such as CERES, MODIS, AMSRE, TRMM, and MOPITT has made it possible to observe climate changes from up above the skies. It is also possible to compare past climate situation with the current one.
- Predicting potential landslides
Landslides cause noticeable death and wealth loss around the globe. INSAR uses inter ferometry remote sensing technique for providing an early warning regarding potential landslides.
Reference Books
- Surveying and Levelling by Kanetkar T. P. Et al
- Basic Civil Engineering by Gopi Satheesh,
