Concept and Applications of Digital Elevation Model (DEM)

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.

 4.  Indian Portal     Bhuvan

CARTOSAT 1 and 2 derived stereo DEM, available for entire India freely. For specification and other details go through Bhuvan website at http://bhuvan.nrsc.gov.in/data/download/index.php

 Uses of DEM

 Digital Terrain Model Generator + Textures (Maps) + Vectors

Common uses of DEMs include:

  • Extracting terrain parameters for geomorphology
  • Modeling water flowfor hydrology or mass movement (for example avalanches and landslides)
  • Modeling soils wetness with Cartographic Depth to Water Indexes (DTW-index) 
  • Creation of relief maps
  • Rendering of 3D visualizations.
  • 3D flight planningand TERCOM
  • Creation of physical models (including raised relief maps)
  • Rectification of aerial photographyor satellite imagery
  • Reduction (terrain correction) of gravitymeasurements (gravimetry, physical geodesy)
  • Terrain analysis in geomorphologyand physical geography
  • Geographic Information Systems(GIS)
  • Engineeringand infrastructure design
  • Satellite navigation(for example GPS and GLONASS)
  • Line-of-sight analysis
  • Base mapping
  • Flight simulation
  • Precision farmingand forestry
  • Surface analysis
  • Intelligent transportation systems(ITS)
  • Auto safety / Advanced Driver Assistance Systems(ADAS)
  • Archaeology

 Source(s): Wikipedia, NPTEL, GIS tutorials

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Surveying : Dumpy Level- Importance and Use

What is Surveying?

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.

Source(s) and Reading(s):

Wikipedia, Step by Step Guide

NIOS site

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Hierarchical Levels of Planning Regions

 

In multi level planning there is certain hierarchy of regions. WE can classify it in many ways depending on chosen criterion/criteria. One such classification on size criterion is here.

 Macro Regions

Macro region is naturally bigger. Macro region can be a state of even a group of states, if the states of a country are not big enough. A Macro-major region can be a zone in a country, which may comprise of a few States. For example, in India there are East, West, North, South and Central Zones and ‘Zonal Councils’ of which function is mutual Consultation, developing cooperation and mutual counseling.In a sense macro regions are second in hierarchy, next to the national level. It is also possible that a physical macro region may comprise parts of different states of a country or project planning purposes. (e.g., big river valley projects, an electric grid of different states, and, for the purpose of a particular activity (facility) planning) the macro region ill be parts of different states. State boundaries are not respected in the sense that the macro region may transcend or cut-across administrative boundaries of the states of a country. A macro region may not be uniform or homogeneous in all respects. It may ave homogeneity in one respect (physical complimentarity) and may have heterogeneity n other respect (administrative boundaries). A macro region should have a common resource base and specialization in that resource base, so that production activities can develop on the principle of comparative advantage based on territorial division of labor.(India has been divided into 11 to 20 macro regions-agro-climate or resource regions).The planning Commission of India would have just 5 zonal councils-Eastern, Northern,Central, Western and Southern comprising of certain states but beyond this there is no macro-regionalization in India.These so-called macro regions of India have to have inter state cooperation in the matter of utilization of river water and electricity grids etc.

 Meso Regions

Meso region can be identified with a ‘division’ of a state. Chattisgarh Region,Bundelkhand Region, Baghelkahand Region, Mahakoshal region is usually a sub-division of a state, comprising of several districts. There should be some identifiableaffinity in the area which may even facilitate planning. It can be cultural or administrative region and it will be even better if it is a homogeneous physical region

Micro Regions

In multi-level planning, district is the micro region. It becomes the lowest territorial unitof planning in the hierarchy of planning regions. The most important reason why districtis the most viable micro region for planning is the existence of database and compactadministration. This is the area, which is viable for plan formulation with administrationfor plan implementation and monitoring.A metropolitan area can be one micro region and the area of influence can be another micro region. A nodal point is also a micro region, though in many cases micro regionsare basically rural areas, which may have a number of minor nodes without anyorganizational hierarchy influencing the entire area. The basic characteristic of a microregion is its smallness. There can be some specific micro regions such as belts of extraction of mineral or a reclaimed area, or a not-so-big command area of an irrigational project.

Micro – Minor Region:

This is the region which is associated with, what is called, the grass-root planning. Amicro-minor region can be a block for which also data exists now and for which theremay be a plan. (So far as the quality of data is concerned, there is hardly any activity, or sector, or region or field for which data is not cooked by the vested interest groups: but,that is another story).The block level plan is integrated with the national plan, through the district and statelevel plans. A block level plan is not surgically cut portion of the district plan, which hasits own logic and linkage.At block level, most of the officers will be more concerned with the implementation of the plans than formulating the plans. At block level, the main exercise will be to take intoaccount of the physical and human resources and to find out the prime moving activitieswhich will enable the block people to make best use of the development potential of the block to meet the basic needs of the people.Minimum needs can be satisfied with the production of basic goods with the help of lowentropy local resources. Yet it cannot be said that ‘external help’ will not be necessary.Infrastructure support has to come from the developed regions. Infact, planning of thedevelopment of the transport, communication, banking, education, medical and manyservice facilities has got to be done at the national level.At the panchayat level, basic goods and services can be arranged through the efforts of the local people. Many activities can be so planned that they improve the socio-economicconditions of the people without being the part of the national plan. Several activities can be undertaken with the cooperation of the local people, with minimum of financial andreal resource support from outside e.g., development of dairying, animal husbandry, pisciculture, poultry, soil conservation measures, optimization of the cropping pattern,

.The most important test of micro-minor planning is that the people need not look towards the centre for it. Now days, a lot of importance is given to ‘water harvesting’.Water is proxy for the use of modern inputs in agriculture. Much of the run-off water goes waste and the infiltration rate is also low. If this water can be harvested, not onlythe run-off water can be stored, but sub-soil water reserves can also become rich. Micro-minor watershed development program probably will be the most important program for a country like India. The optimum land use planning can start from the micro-minor area only

 Source::  PLANNING EFFORTS IN INDIA SINCE
INDEPENDANCE
ADITYA A. PUNAGAVKAR. 
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Hierarchical Levels of Planning

The hierarchy of public activity planning and regional development planning is by and large based on a functional division of labor between the different levels. At any level types of plans physical planning shares more extensively similarities both in substantial contents and procedures. The practical implication is that there will be some kinds of overlapping between planning levels, at least when it comes to plans under local planning authority. However, some overlapping might occur in the division between planning at the central state tire as well as regional and local levels.

There are number of plans available to direct the development in an urban or rural areas. The levels at which they operate is different & similarly the nature and details provided in each type is different. Hierarchy and Types of Plans are explained below

Hierarchy of Plans

Master plan :

A coordinated act of planning proposals, for the physical development of a city, via the purposeful transformation of its socio-economic, natural and built environment, taking into consideration the existing requirements and the future needs , with population as the basic parameter.

 Contents of the master plan

  • extent of planning area
  • immediate surrounding area and its effects
  • broad delineation of the land use
  • major circulation pattern of the city
  • major work centers
  • delineation of high and low population density zones
  • zone and sub zone divisions
  • development codes and norms
  • allocation o land for various use zones
  • policies and proposals for development

 Main functions of the plan :

  • to develop the town or city as a combined unit and maintain a balance b/w the spatial allocations for the distribution of facilities
  • formulation of policies for the development of the town/city, aiming at the decentralisation of city centre
  • presenting broad circulation links, for inter-city & intra-city traffic and a multi modal mass transport system
  • Preservation of the natural features of the city
  • division of the city in sub-divisions or zones.

 Zonal development plans :

The master plan divides the city into sub-divisions or zones

Criteria followed are :

  1.          physical & historical growth
  2.          character of land
  3.          intensity of land – use
  4.          circulation pattern ( railways , major arteries etc. )
  5.          municipal boundaries , election & census wards

Contents of a zonal plan

  • land use plan confirming to the master plan
  • location and extent of land uses
  • more detailed circulation pattern
  • special objectives of the zone if any
  • allocation of use zones into further use premises

Functions of a zonal plan

  • A zonal development plan details out and elaborates the policies of the master plan
  • Acts as a link between the master plan and the layout plans
  • Contains a land-use plan for the development of the zone and show the approximate locations and extents of land-uses proposed in the zone
  • The schemes and layout plans indicating use premises should confirm to the master plan

Local area plan

A local area plan (lap) sets out a strategy for the proper planning and sustainable development of a specific area within a local authority and for a timescale as specified by the authority.

Contents of a local area plan

  1. Land use zoning & density
  2. Public open space
  3. Private open space
  4. Car parking
  5. Provision of infrastructure
  6. Conservation of built heritage
  7. Conservation of natural environment
  8. Provision of traveller accommodation
  9. Community facilities
  10.   Design & development standards.

Functions of a local area plan

  • a local area plans gives plot level detail
  • it is also used to check if the master plan is confirming with land.

A sector plan consists of a group of neighborhoods where it is possible to provide higher order facilities for larger population

Contents of a sector plan

  • it is a detailed site plan with broad identification of residential clusters
  • Allocation of commercial areas and other facilities based on access requirement
  • Formation of a boundary depending on circulation pattern  and administrative setup
  • Social and physical infrastructure to be allocated based on development control norms laid down in master plan
  • Traffic links to be identified between arterials and collector roads

Functions of a sector plan

  • Each sector plan has to identify the various neighborhoods with population ranging from 3500-15000
  • It is the lowest level plan for the implementation of the various levels of planning proposals extensively detailed out

hierarchy and types of plan

 

Source(s): Planning Tank

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