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An Integrated Rural Mapping using
Remote Sensing and GPS Techniques
Global Positioning System
This space-based technique is widely being used these days in association with other mapping technologies. Details of this technique are given in the next section.
Laser Range Finder
These are devices, which tries to overcome the disadvantages, GPS and other surveying techniques have. GPS may not work in highly dense area or with more than 20-30% canopy. However, there are GPS available, which works even in more than 30% canopy. The advantages with the laser range finders and data loggers are that they are hardware and operating system independent. Any hand-held computers can be used for collecting and storing data from these devices. However, at present, they have not made their presence felt due to cost factor, but with time, they will be available at a fraction of cost to its present cost, and it will probably be widely used for mapping for GIS projects.
Following table gives a comparison of the accuracy level of different mapping methods available,
Instrument Typical error
| Instrument |
Typical error |
| Distance |
Horiz. Angle |
Vert. Angle |
Elevation |
| Distance by tape (fiberglass) |
+ 1% * |
|
|
|
| Distance by stadia |
+ 0.33% |
|
|
|
| Brunton compass |
|
Maybe + 1° * |
|
|
| Plane table and Alidade |
Similar to tape and compass mapping |
Similar to Autolevel |
| Autolevel |
(stadia or tape) |
+ 1° * |
|
+ 0.001 to 0.005 m # |
| Theodolite |
|
+ 1° - 30" |
Same as horizontal? |
+ 5 mm # |
| Total station |
+ 5 - 10 mm at 200 m # |
+ 1" - 10 " # |
Same as horizontal? |
Maybe + 2 mm * |
| GPS |
+1 cm withpost-processing |
|
|
+ 5 cm with post-processing |
| Laser Range Finder |
+ 0.15 - 0.20 m # |
+ 0.5° # |
+ 0.2° # |
Depends on distance and vert. angle accuracy |
Other advantages and disadvantages of the mapping methods can be listed as,
| Mapping technique |
Range |
Power needed? |
Portability |
Ease of set-up and operation |
Electronic data storage and downloading? |
| tape and compass |
Typically 200-300 m * |
No |
very high |
High |
No |
| plane table |
Typically 100-200 m * |
No |
High |
High |
data stor. yes
downld. no |
| Autolevel |
Typically 100-200 m * |
No |
High |
High |
No |
| transit/theodolite |
Typically 200-300 m * |
No |
moderately high |
Moderate |
No |
| total station |
500 to >2500 m |
Yes |
Moderate |
Low |
Yes |
| GPS |
nearly global |
Yes |
High |
Moderate |
Yes |
| laser rangefinder |
500 m |
Yes |
High |
High |
yes with add-ondata collector |
Source:
http://geography.uoregon.edu/mcdowell/EM&M/surv.html
* Estimate in actual use
# Manufacturer's estimate of precision/optimal accuracy
Use of GPS in Surveying
GPS (Global Positioning System) is a worldwide, all weather, 24-hours navigation and timing system. The position derived form the constellation of 24 operational satellites is of very high accuracy on a reference frame called WGS-84. The accuracy of the derived positions varies with the type of instrument used for collecting data, method used in the surveying, post-processing done and the method of post-processing. The accuracy varies from few mm to several meters. With the removal of S/A (Selective Availability), available prior to May 2000, the accuracy of GPS receivers has increased greatly, in autonomous mode. This has prompted the users and GPS receiver manufacturers to come up with low cost solutions for GPS surveying. The users have started using GPS for low cost mapping and manufacturers have started innovating and coming up with miniature GPSs (Satyaprakash, GIS@development, August 2002). With the availability of coast guard beacons, all over the coast, the differential corrections are available to the users, all over the world, for free. This has eliminated the need of another GPS receiver for better accuracy. Also real time differential corrections are available, at a nominal price, world over, through satellites. These factors have increased the uses of GPS in almost all walks of life.
Another innovation is the integration of GPSs with mobile devices, like PDAs (Personal Digital Assistants), Cell phones etc. More and more people have started using GPS for locating themselves in real time and transmitting their location for LBS (location based services). There have also been advancements in the field of integration of GPS with other surveying devices, like TS (Total Station), LRF (Laser Range Finder) and Camera. The developments in the field of data collecting devices and miniaturization of GPS receivers has led to the use of GPS and data loggers for common mapping practices for collection of spatial and attribute data for GIS.
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