How do navigators use the stars, including our sun, the moon, and planets to find their way? Well, for at least two millennia, navigators have known how to determine their latitude — their position north or south of the equator. At the North Pole, which is 90 degrees latitude, Polaris (the North Star) is directly overhead at an altitude of 90 degrees. At the equator, which is zero degrees latitude, Polaris is on the horizon with zero degrees altitude. Between the equator and the North Pole, the angle of Polaris above the horizon is a direct measure of terrestrial latitude. If we were to go outside tonight and look in the northern sky, we would find Polaris at about 40 degrees 13 minutes altitude – the latitude of Coimbra.
In ancient times, the navigator who was planning to sail out of sight of land would simply measure the altitude of Polaris as he left homeport, in today’s terms measuring the latitude of home port. To return after a long voyage, he needed only to sail north or south, as appropriate, to bring Polaris to the altitude of home port, then turn left or right as as appropriate and “sail down the latitude,” keeping Polaris at a constant angle.
The Arabs knew all about this technique. In early days, they used one or two fingers width, a thumb and little finger on an outstretched arm or an arrow held at arms length to sight the horizon at the lower end and Polaris at the upper.
The critical development was made independently and almost simultaneously by John Hadley in England and by Thomas Godfrey, a Philadelphia glazier, about 1731. The fundamental idea is to use of two mirrors to make a doubly reflecting instrument—the forerunner of the modern sextant.
How does such an instrument work? How many of you have ever held a sextant in your hand? Hold the instrument vertically and point it toward the celestial body. Sight the horizon through an unsilvered portion of the horizon mirror. Adjust the index arm until the image of the sun or star, which has been reflected first by the index mirror and second by the silvered portion of the horizon mirror, appears to rest on the horizon. The altitude of the heavenly body can be read from the scale on the arc of the instrument’s frame.
Hadley’s first doubly reflecting octants were made from solid sheets of brass. They were heavy and had a lot of wind resistance. Lighter wooden instruments that could be made larger, with scales easier to divide accurately and with less wind resistance quickly replaced them.
Hadley’ octant of 1731 was a major advancement over all previous designs and is still the basic design of the modern sextant. It was truly a “point and shoot” device. The observer looked at one place – the straight line of the horizon sighted through the horizon glass alongside the reflected image of the star. The sight was easy to align because the horizon and the star seemed to move together as the ship pitched and rolled.
We have seen how navigators could find their latitude for many centuries but ships, crews and valuable cargo were lost in shipwrecks because it was impossible to determine longitude. Throughout the seventeenth century and well into the eighteenth century, there was an ongoing press to develop techniques for determining longitude. The missing element was a way to measure time accurately. The clock makers were busy inventing ingenious mechanical devices while the astronomers were promoting a celestial method called “lunar distances”. Think of the moon as the hand of a clock moving across a clock face represented by the other celestial bodies. Early in the 18th century, the astronomers had developed a method for predicting the angular distance between the moon and the sun, the planets or selected stars. Using this technique, the navigator at sea could measure the angle between the moon and a celestial body, calculate the time at which the moon and the celestial body would be precisely at that angular distance and then compare the ship’s chronometer to the time back at the national observatory. Knowing the correct time, the navigator could now determine longitude. When the sun passes through the meridian here at Coimbra, the local solar time is 1200 noon and at that instant it is 1233 PM Greenwich Mean Time. Remembering that 15 degrees of longitude is equivalent to one hour of time gives us the longitude of 8 degrees, 15 minutes West of Greenwich. The lunar distance method of telling time was still being used into the early 1900’s when it was replaced by time by radio telegraph.
An octant measures angles up to 90 degrees and is ideally suited for observations of celestial bodies above the horizon. But greater angle range is needed for lunar distance observations. It was a simple matter to enlarge Hadley’s octant, an eighth of a circle, to the sextant, a sixth of a circle, that could measure up to 120 degrees.
In the first half of the eighteenth century there was a trend back to wooden frame octants and sextants to produce lighter instruments compared to those made of brass.
Probably the finest 18th century instrument maker was the Englishman Jesse Ramsden. His specialty was accurate scale division. Here’s a small brass sextant that Ramsden made shortly before his death in 1800. Ramsden’s major achievement was to invent a highly accurate “dividing engine”—the apparatus used to divide the scale into degrees and fractions of degrees. His design was considered so ingenious that the British Board of Longitude awarded Ramsden a prize of 615 pounds—in 18th century terms, a small fortune. His “dividing engine” now resides in the Smithsonian Institution in Washington.
The development of more precise scale division was a milestone in instrument development. Certainly, it permitted more accurate observations but it also permitted smaller, lighter, more easily handled instruments.
चक्रवात
******
– वे निम्न वायुदाब के केन्द्र जिसके चारों ओर बढ़ते हुये वायुदाब की समदाब रेखायें होती हैं। चक्रवात में
पवन की दिशा-परिधि से केन्द्र की ओर हेती हैं।
– उत्तार-गोलार्द्ध में यह हवा घड़ी की सुई की दिशा के विपरित गति करती हैं। ऐसा पृथ्वी की घूर्णन
गति के कारण होता हैं और दक्षिणी गोलार्द्ध में घड़ी के अनुमूल चलती हैं।
– चक्रवातों का आकार अण्डाकार, गोलाकार या V.आकार का होता हैं।
चक्रवात दो प्रकार के होत हैं।
(1) शीतोष्ण-कटिबंधीय चक्रवात:-
यह अण्डाकार, गोलाकार या ट के आकार के होते हैं। जिसके कारण इन्हे लो गर्त/ट्रफ कहते हैं। शीतोष्ण
कटिबंधीय चक्रवता का व्यास 1920 किलोमीटर होता हैं। कम से कम व्यास 1040 किलोमीटर होता हैं। कभी-
कभी ये चक्रवात 10 लाख वर्ग किलोमीटर तक फैले जाते हैं। ऐसे चक्रवात 35-65 अक्षांशों के मध्य
दोनो गोलार्द्ध के पाये जाते हैं। पछुआ पवनों के कारण ये पश्चिम से पूर्व दिशा…
A digital elevation model (DEM) is a digital model or 3D representation of a terrain’s surface — commonly for a planet (including Earth), moon, or asteroid — created from terrain elevation data.
In most cases the term digital surface model represents the earth’s surface and includes all objects on it. In contrast to a DSM, the digital terrain model (DTM) represents the bare ground surface without any objects like plants and buildings.
DEM is often used as a generic term for DSMs and DTMs, only representing height information without any further definition about the surface. Other definitions equalise the terms DEM and DTM, or define the DEM as a subset of the DTM, which also represents other morphological elements. There are also definitions which equalise the terms DEM and DSM. On the Web definitions can be found which define DEM as a regularly spaced GRID and a DTM as a three-dimensional model (TIN). Most of the data providers (USGS, ERSDAC, CGIAR, Spot Image) use the term DEM as a generic term for DSMs and DTMs. All datasets which are captured with satellites, airplanes or other flying platforms are originally DSMs (like SRTM or the ASTER GDEM). It is possible to compute a DTM from high resolution DSM datasets with complex algorithms. In the following the term DEM is used as a generic term for DSMs and DTMs.
Types of DEM
Height map of Earth’s surface (including water and ice) in equirectangular projection, normalized as 8-bit grayscale, where lighter values indicate higher elevation.
A DEM can be represented as a raster (a grid of squares, also known as a heightmap when representing elevation) or as a vector-based triangular irregular network (TIN). The TIN DEM dataset is also referred to as a primary (measured) DEM, whereas the Raster DEM is referred to as a secondary (computed) DEM. The DEM could be acquired through techniques such as photogrammetry, lidar, IfSAR, land surveying, etc. DEMs are commonly built using data collected using remote sensing techniques, but they may also be built from land surveying. DEMs are used often in geographic information systems, and are the most common basis for digitally produced relief maps. While a DSM may be useful for landscape modeling, city modeling and visualization applications, a DTM is often required for flood or drainage modeling, land-use studies, geological applications, and other applications.
Representation of Elevation Data
2. Raster (Example GRID and ASCII), which could be square, rectangular, hexagonal, triangular in shape) (GRID and ASCII stands for “Generic Region for Information Display” and “American Standard Code for Information Interchange” respectively)
2. Vector (Example TIN, which is triangular only and stands for “Triangulated Irregular Network”).
Production
Mappers may prepare digital elevation models in a number of ways, but they frequently use remote sensing rather than direct survey data. One powerful technique for generating digital elevation models is interferometric synthetic aperture radar where two passes of a radar satellite (such as RADARSAT-1 or TerraSAR-X or Cosmo SkyMed), or a single pass if the satellite is equipped with two antennas (like the SRTM instrumentation), collect sufficient data to generate a digital elevation map tens of kilometers on a side with a resolution of around ten meters. Other kinds of stereoscopic pairs can be employed using the digital image correlation method, where two optical images are acquired with different angles taken from the same pass of an airplane or an Earth Observation Satellite (such as the HRS instrument of SPOT5 or the VNIR band of ASTER).
Older methods of generating DEMs often involve interpolating digital contour maps that may have been produced by direct survey of the land surface. This method is still used in mountain areas, where inter-ferometry is not always satisfactory. Note that contour line data or any other sampled elevation datasets (by GPS or ground survey) are not DEMs, but may be considered digital terrain models. A DEM implies that elevation is available continuously at each location in the study area.
The quality of a DEM is a measure of how accurate elevation is at each pixel (absolute accuracy) and how accurately is the morphology presented (relative accuracy). Several factors play an important role for quality of DEM-derived products:
terrain roughness;
sampling density (elevation data collection method);
grid resolution or pixelsize;
interpolationalgorithm;
vertical resolution;
terrain analysis algorithm;
Reference 3D products include quality masks that give information on the coastline, lake, snow, clouds, correlation etc.
Methods for obtaining elevation data used to create DEMs
Lidar
Stereo photogrammetryfrom aerial surveys
Structure from motion/ Multi-view stereo applied to aerial photography
Block adjustment from optical satellite imagery
Interferometry from radar data
Real Time KinematicGPS
Topographic maps
Theodoliteor total station
Doppler radar
Surveying and mapping drones
Free Data sources
1. Space Shuttle Radar Topography Mission (SRTM)
This 1-arc second global digital elevation model has a spatial resolution of about 30 meters covering most of the world with absolute vertical height accuracy of less than 16m. SRTM DEM data is being housed on the USGS Earth Explorer server.
2. ASTER Global Digital Elevation Model
A joint operation between NASA and Japan was the birth of Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER). ASTER GDEM boasted a global resolution of 90 meters with a resolution of 30 meters in the United States. Despite its high spatial resolution and greater coverage (80% of the Earth), users were dissatisfied with it because of its artifacts, which often occurred in cloudy areas. You can download the ASTER DEM data for free from the “USGS Earth Explorer”.
3. JAXA’s Global ALOS 3D World
The ALOS World 3d is a 30-meter spatial resolution digital surface model (DSM) constructed by the Japan Aerospace Exploration Agency’s (JAXA). Recently, this DSM has been made available to the public. It is the most precise global-scale elevation data at this time using the Advanced Land Observing Satellite “DAICHI” (ALOS). The DSM was generated using stereo mapping (PRISM) for worldwide topographic data with its optical stereoscopic observation. In order to obtain this highly accurate DSM, you’ll have to register online through the “JAXA Global ALOS portal” to download it.
Knowing about the relative position of various objects, determination of distances between them, measurement of angles, measurement of height, determination of boundaries and relative heights of various points come under the purview of surveying. It is very essential to mark the various points on the land, boundary lines of the proposed construction sites and levels (heights) of the various locations before starting the construction of building, bridge, embankment, railway line etc. After performing the measurements of the shape, size and location of objects on ground, the details are plotted on paper (drawing sheet). After the completion of drawing which may be one or more than one, describing the details, the construction process is started. After determining the details of the soil strata below ground and bearing capacity of soil which is called Geotechnical survey, the depth of earth work is decided by the engineer. All these things are interrelated and forms the part of the total survey work. At first sight the job of survey looks very simple but it actually requires special knowledge about it and the understanding of the job. The students who have studied or who have the knowledge of Mathematics and Physics can acquire the knowledge about survey in a nice manner. The pace of development which is going on in India and through out the world has increased the importance of survey, related works. Measurement of Length.
LEVELLING
Leveling (survey) is conducted either by a conventional apparatus called “Dumpy level” or by a modern apparatus called “Total station” (Automatic modern levelling apparatus). In both the machines a telescope is horizontally mounted on a tripod stand. This telescope is free to move in 360o rotation in the horizontal plan. The viewing glass near the eye is called eye piece, and the viewing glass facing the object is called object piece. The line (imaginary) which joins the centres of the eye piece and object lens, is called the line of collimation. Dumpy level In the Dumpy level the line of collimation is made horizontal by using the bubble of the sprit level which comes into middle when the horizontality is achieved. This is done by using leveling screens. In automatic leveling machine a rough horizontality is achieved by using spirit level and finally fine tuning is done in an automatic manner. After achieving, the horizontality of line of collimation The instrument is turned by 360 degrees . The imaginary plane formed by the rotation of line of collmation by 360o is called the plane of col
Dumpy Level
A dumpy level (also known as a Builder’s Level) is an optical instrument used to establish or check points in the same horizontal plane. It is used in archaeological surveying to measure horizontal levels, for example to demonstrate the difference in height at the top and base of a slope such as an excavated pit or a surviving earthwork.
In 1832, English civil engineer William Gravatt, who had worked with Marc Isambard Brunel and his son Isambard on the Thames Tunnel, was commissioned by Mr. H.R. Palmer to examine a scheme for the South Eastern Railway’s route from London to Dover. Forced to use the then conventional Y level during the work, Gravatt devised the more transportable and easier to use dumpy level
Equipment-> The level ‘kit’ consists of a level head , staff and tripod. The level head comprises an eyepiece, bulls eye spirit level, three levelling screws and a focus for the telescope lens; the base also incorporates a 360 degree compass. The 5m staff is in sections. Each ‘block’ represents one centimetre, and each ‘E’ represents 5 centimetres. The 10 cm sections alternate back and forth and between black and white, and the colour alternates between black and red for each metre. The tripod is composed of aluminum and plastic, with three extendable/lockable legs and a base plate with screw fitting with which to attach the level head. There is a canvas carrying strap and a belt to secure the legs together. Benchmarks and Temporary Bench Marks (BM/TBM) Find the nearest OS Bench Mark (BM), which is part of the national height system for mainland Great Britain and forms the reference frame for heights above mean sea level. Bench Marks are no longer maintained by the Ordnance Survey (although Fundamental (F)BMs are), but they should be marked on most maps. If the height value is not shown on the map Bench Marks can usually be found on churches, but also on other notable buildings, houses, bridges etc. The database describes where it is and what type of benchmark symbol is used (usually carved into stone, the centre of the horizontal groove is the height reference). It is worth finding the nearest BM to your survey site as soon as possible so that you can establish the best way to transfer the height from the BM to your If there is no BM nearby to your site you can establish a Temporary Bench Mark (TBM) at an arbitrary height, for example 100m (to ensure all heights are positive). At some point you will also need to find the nearest BM, to tie your TBM into and then make your final level calculations. To set up a TBM: mark an easily identifiable permanent feature nearby – eg. a coloured brick in a wall (as in the photograph right), or a fence post; a wooden stake may also be used but check with the landowner (if it is a scheduled monument this is not an option). Make a careful note with a precise description describing the location and nature of the TBM, preferably with a note annotated on a map and a digital photograph (if you have a handheld GPS use this!)
Setting up the level
Set up the tripod where you have a clear sight of the benchmark, at a similar height to but preferably higher, than the benchmark. If possible, set up in the centre of the area that you intend to survey, or somewhere that you can see all of the site as well as the backsight/Bench Mark, with the top plate relatively level. Release the catches on each leg and extend to full length, close the catches. Space the tripod legs well apart, with the level plate about chest height of the person who will be reading the levels. NB: the tripod needs to low enough for the smallest person on site to use the dumpy level! Place the level head on the baseplate and attach it to the central screw beneath the base plate. With the telescope parallel to two of the foot screws, level off by adjusting the two foot screws simultaneously, turning them in opposite directions until the level bubble is central. Then turn 90 degrees so the telescope points towards the third foot screw, and use the third screw to adjust the spirit level until the bubble is central along this axis. Check again in all directions. Now you should be perfectly level.
Taking a reading
Taking the backsight (BS)
The first measurement that you need to take is the backsight. This will enable you to calculate the height of the instrument/level (IH) from which all other levels are calculated.
The person with the staff should place the bottom of the staff level on the BM or TBM, keeping it as vertical as possible.
The person at the Level rotates the telescope until the central line/cross hairs are lined up with the staff; you may need to bear in mind that if at some point you have to move the level (higher or lower, or to a new location) you will need to re-level it and retake the backsight reading
Focus the eyepiece first to see the cross hairs then the telescope focus to see the numbers on the staff; use the fine adjustment to be perfectly lined-up.
When looking through the telescope, you take the reading where the central or stadial cross hairs meet, to the nearest centimetre. For example in the diagram to the right (above) the reading would be 1.42m. The levelling bubble should be central
Calculating the instrument height (IH) (or height of the Level) In order to calculate the height of the instrument (IH; ie the height of the Level Head telescope) you add the value of the reading you have just taken to the known value of the BM or TBM that you are using. 3.
Taking Foresight(FS) readings and calculating reduced levels Begin taking height (level) readings of anything you want to illustrate on your site: top of slope, bottom of slope, break of slope – to illustrate changes in height and create profiles. Mark the location of your levels on the plan, starting at 1or the next available number if returning to a survey, and read off each height reading and record these in a separate notebook. Make sure you write clearly and record the date, where the survey is, what the BM or TBM is and the initials of the people undertaking the survey. 3 eyepiece focus eyepiece instrument focus baseplate screw and plumbob hook 360° base levelling screws instrument rotate Each time you will have to rotate the telescope, sight on the staff in its new location, focus and carefully take the reading, always check twice that you have read the number correctly Once you have taken all the levels you want, you will need to calculate the actual height values, or reduced levels (RL) by subtracting each one from the instrument height (IH). This gives you the ‘real’ height of the ground at the base of the staff. Example Suppose The Bench Mark value was 8.52m, so a TBM was created on the concrete at the base of the wall, which was 0.7m below the height of the BM, giving the height of TBM 1 of 7.82m. This was then used to backsight to, which gave a reading of 0.3m, which added to the TBM gave an IH of 8.12m (TBM1 + BS = IH). A foresight was then taken on the new TBM, this reading was 0.16m, which subtracted from the IH gives a value of 7.96 for TBM 2 (IH – FS = TBM2). BM= 8.52 – BS = -0.7 TBM 1 = 7.82m + BS= 0.30 IH = 8.12 – FS = 0.16 TBM 2 = 7.96m The value of TBM 2 was 7.96m, the new backsight reading was 0.29m, giving an instrument height of 8.25m (TBM + BS = IH). A number of foresights were then taken within the survey area and these readings were then subtracted from the instrument height to give a real/reduced level. TBM 2 = 7.96 + BS = 0.29 IH = 8.25 (IH – FS = RL) Fore sights (m) Reduced Level (m) 1 = 2.25 6.00 2 = 3.62 4.63 3 = 1.92 6.33 4 = 2.18 6.07 5 = 3.19 5.06 6 = 3.09 5.16 4
Points to Remember
The place of which height is to be measured is called Station.
Height is always measured with reference to sea level.
Survey of India established benchmarks (B.M) at several places.
Ideally the distances should be taken from the benchmark.
If it is not available then we can select point on the map whose distance from sea level is known as the reference.
We can fix any suitable point as Temporary bench mark and all heights can be measured from that point. We can fix any temporary bench mark , but if its MSL( at temporary BM) is not sure , then before starting the surveying permanent BM reading should be clear.
• Now we will learn how to take actual readings. Please observe below figure carefully .
Some notes on taking care and use of the equipment
The staff can be difficult to steady in high winds; you do need to keep it vertical and still. Do not use fully-extended near overhead power cables.
Always pull out (and return) the sections one at a time, and put the staff back in its sleeve after use. Keep mud and grit off it as much as possible, as this will scratch the painted markings. After a survey, dampen the microfibre cloth supplied and wipe off each section of the staff as you close it up.
The level head is a precision instrument, and should be handled carefully. When not in use it should always be kept in its box. If it is raining please make sure that you cover it with a bag or rain hood, or preferably unscrew the head and place it in its box. If the level head does become wet, make sure that it dries out somewhere inside/out of the rain before being returned to its box. If you don’t dry it out properly, moisture may seep inside which will result in the telescope ‘fogging up’ and possible damage to the internal parts.
When your survey is complete, carefully unscrew the level head and place it in its correct position within the box, close the lid and make sure the catch is secure.
PLEASE TAKE CARE NOT TO DROP THE BOX: If the level head or box containing the level head is kicked or dropped you must report this to the Jigsaw team as soon as possible as it may require calibration or repair.
Undo the catches on the tripod legs and carefully move the retract the legs and clamp the catches back and fasten the belt. Please ensure that the tripod does not get dented or damaged, as this may make it unusable.
