An Integrated Rural Mapping using
Remote Sensing and GPS Techniques
Dr. Satyaprakash and Dr. R. Sivakumar GIS Institute CSDMS, G-4, Sector-39, Noida Abstract Of late, GPS has been increasingly recognized as one of the best surveying technology. It has found numerous applications in surveying for sub-centimeter level accuracy like, from crustal deformation and missile launching to coarse surveying for sub-meter accuracy such as geological mapping and locating fishes. The measurement from GPS is used in getting locations for GIS projects and also for the spatial distributions of GCPs (Ground Control Points) for georeferencing in digital image processing of satellite images and for high-resolution image analysis. This paper discusses, the use of GPS and its integration with high-resolution images for different kinds of surveying/mapping, with emphasis on its use with low cost mapping in the rural sector. The paper takes up a case study of a village in Faridabad. Introduction Rural mapping is as old in India as is the culture of revenue collection or “Lagaan” collection. Even before that, people used to have record of their lands, both in the form of maps and in the tabular form, for ownership purposes. Shershah Suri, an eminent ruler first gave a methodical shape to the land record survey system, in north India, for a better way of collecting his land revenues. The British continued to operate with the same system of land records for land revenue collection. However, for the government, it is not only the land revenue collection that is important but also the development of villages. For proper development, government needs various critical information (spatial and non-spatial) in the form of maps of the villages, agricultural area, socio economic data, the settlements that are scarcely available, etc. Different surveying techniques have been used in the past for mapping the land of the villages. Each technique has its own merits and demerits, and it has been often rightly found that not one technique is able to give a complete solution to the problem of mapping the villages. With the advent of space based technologies, like GPS, Remote Sensing and Aerial Photography, it has become easier for implementers to integrate the data collected through these technologies with a GIS (Geographic Information System) for better governance. Different Surveying for Rural Mapping Different surveying techniques are being used and have previously been used for rural mapping. It has been found that none of them suffice the need of modern technology. The most commonly used can be listed below. Traditional Traditional survey includes the measurement of distances by tape, stadia, angles by compass, mapping by plane table survey, leveling by autolevel, angle measurement by theodolite, distance measurement by EDM (Electronic Distance measurement) and the most recent one TS (Total Station), which is a combination of theodolite and EDM. But surveys by these traditional methods are labour intensive and time consuming. With time, there has been a need of integrating these data and maps to the GIS, which is again time consuming and labour intensive. Aerial Photography This technique is relatively more accurate and quick. One can generate 1:4000 or even larger scale maps using a 1:20,000 photograph with modern digital photogrammetry techniques. However, due to security issues, in India, this technique is not that popular and not available to the common man. In spite of the restrictions, some areas have been mapped using Aerial photography and such photographs are available. Satellite Remote Sensing The recent advances in space technology have the potential of identifying the sub meters of an object on the ground. Hence in rural mapping the high-resolution images like IRS – PAN with the resolution of 5.8 meter is being used to identify the rural road network and update the boundary information. Similarly the launching of IKONOS data with the 1mt spatial resolution and 4mt multispectral data is widely used in mapping even a small hut in a village level. The analysis of the satellite data involves both, visual interpretation of image and digital image processing techniques. The accuracy point of view one can prepare map unto 1:5000 scale using Geoproducts of Ikonos. Several studies have been conducted on the aspects of accuracy due to integration with aerial photography and global positioning system. After pre-processing the raw data, the images are interpreted with image interpretation elements. But due to limitation in visually identifying the features it requires ‘digital image processing’ to enhance the interpretation capability through various processing techniques like image enhancement, filtering, band ratioing etc., 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
Other advantages and disadvantages of the mapping methods can be listed as,
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. Types of GPS Surveying The GPS, from its inception, has been a surveying and locating instrument. It has found application in different fields, and even in surveying, for different kind of surveying. A few of the applications of GPS in the field of surveying are described in the forthcoming paragraphs. Geodetic surveying: Dual frequency GPS receivers, in differential mode, have been in use for quite a long time towards geodetic surveying for construction, highway surveying, mine surveying, crustal deformation studies, to name a few. This type of surveying requires sub-cm level accuracy, and in some cases, even of the order of mm level accuracies. Such an accuracy is achieved through use of dual frequency receivers (capable of receiving both L1 and L2 signals), carrier phase post-processing of GPS observables (achieved through post-processing scientific software like Bernese, GAMIT, GYPSY, and vendor specific software). Special care is taken in this case for establishment of base station for the survey, and atmospheric modeling during the post-processing. These care lead to better ambiguity resolution for better accuracies. Advancements in the data processing techniques in the recent past have increased the accuracy to a considerable limit. GIS Mapping: Single frequency code measurement GPSs and availability of carrier phase measurement facility on these single frequency GPS receivers has led to the entry of GPS for spatial data collection for the GIS projects. These receivers are of medium accuracies, of the order of sub-meter to few meter accuracy levels, and are being used in almost all GIS project, these days. The use of GPS has virtually removed the conventional surveying methods being used in the GIS projects. This has become possible due to the decrease in the price of GPS receivers and the removal of S/A. Another factor due to which these receivers are being widely used is the availability of coast guard beacons all along the coast. With the availability of differential corrections, through these beacons, the need of an additional GPS receiver has almost been eliminated and real time corrections can be applied to the data collection process for better accuracies. Another advancement in this field is the availability of post processing software for the single frequency receivers, which were not available, some time back. Now, the cheap and robust GPS like GARMIN can also be used as a base station as well as rover for data collection through RHINO (http://www.uspositioning.com) and the position can be resolved with an accuracy of few cm. These have eliminated the use of costly GPS receivers and data processing software. Mobile Mapping: The miniaturization of GPS receivers and computing devices has evolved the technique of mobile mapping. In this technique, any mobile computing device, like palm-top, PDAs and laptops, running a mobile GIS software, is connected to a mobile GPS like Pretec card GPS with or without external antenna. The mobile GIS software can be any like ArcPAD from ESRI (http://www.esri.com), Onsite from Autodesk Inc. (http://www.autodesk.com), HGIS from Stalpal Inc. (http://www.starpal.com), to name a few. This software work with most popular GPS receivers which support the data transfer through NMEA protocol version 2.0 or later and Trimble TSIP protocol through RS232 port. In addition to these commercially available softwares, few free software are also available like Gramchitra, developed by Media Las Asia at CSDMS (http://www.csdms.org). The software Gramchitra, is a LINUX based mobile GIS software available on a open platform and performs most of the functions of popular mobile GIS software through an innovative algorithm. The base map of the area can be loaded at the background, over which the GPS and other spatial data could be collected through an easy to use user interface. The data is stored in its own proprietary format and can be exported to the popular formats like ArcView .shp and MapInfo .tab. GPS for Rural Mapping: A case study At the GIS Institute, CSDMS, the utility of low cost mapping using GPS and its integration with satellite imagery, without sacrificing the accuracy, was explored. The work was initiated in a village, named Riwajpur, in Faridabad District of Harayana State, India. The village has about 120 households and with 600 population. The settlement area of the village extends for about 1.5sq km (Figure) . The existing map of the village was prepared by land survey department in 1953 and there was no updation in the map till date. Hence to update the map with low cost technology, in the first stage, GARMIN make e-trex GPS receiver and Casio make Protrek watch-cum-GPS receiver was used. To get the skeleton map of the village, the streets of the village was mapped, using both GPSs in continuous mode and compared. The track map of the village Riwajpur is shown in figure. The important places, in the village, were also marked on the map.. The IRS-PAN image was georeferenced using the co-ordinates obtained using the same GPS and digitally processed. This street map was then overlaid on the 5.8m resolution satellite image. The roads, as visible on the satellite image, were then interpreted. The figure shows the map generated from the satellite image and also image interpretation was carried out to identify other features. Further, this map was corrected, after ground verification and final map was generated. Figure shows the final corrected and updated map. Ground verification in the measurements shows that there is a 5mts error in the horizontal. Hence further processing with little higher accuracy GPS was used for further research. The process was later refined with Pretec GPS and PDA running ArcPAD. The IKONOS 1mt PAN was merged with 4mt MSS data to get the better spatial and spectral resolution using different image processing techniques. Subsequently the image was georefenced using Pretec GPS reading and was used as the background image for the mapping. The track and the house corners were marked on the image, in the field itself. This expedited the survey process and map making. There was no need of any later ground verification and post-processing of the data. The attribute and the map could be generated simultaneously, with reasonably good accuracy. Figure shows the map created using ArcPAD for the same village. Discussion The above-discussed procedure is good for mapping in the rural areas but there are few inherent problem areas and issues, which need to be understood before undertaking such a work. Problems with Remote sensing procedure Due to various policy restrictions, it is very difficult to get any high-resolution image/aerial photographs of rural areas and also aerial photo is a not available for all rural areas. The time taken by the data supplying agencies is very high for fresh acquisition or supplying the achieved data. The recent quotation for IKONOS Geoproducts is cost about $30 per square kilometer (www.spaceimaging.com) and minimum order should be 49sq.km. The data have to be processed with the specific image processing software for geo correction and post processing. Even though the cost of the IRS-PAN data is very less compare to IKONOS the information on house level cannot be mapped. This is an important parameter while generating any database. The Geoproduct of IKONOS 1mt resolution data has the horizontal accuracy of ± 50mts. Hence to prepare high accuracy map it requires precision products which is much costlier. The geometric distortion is also high with these images. Problems with GPS surveying procedure The GPS used in the study was a single frequency GPS whose accuracy is 30m in stand-alone mode. However, it can be increased with better surveying techniques and post-processing methods. Also the GPS sometimes, does not work in the densely populated areas. Another factor, which need to be kept in mind, is the accuracy for rural mapping. Since the area is very small and the map generated is of the order of 1:2000 or less, the base map used need to be georeferenced using the GPS, being used for mapping. Another factor is the datum. Since GPS gives the data on WGS84 and the maps and images available are on modified Everest ellipsoid, the transformation parameter need to be known, before the GPS data is transformed to appropriate datum. Since the transformation parameter is classified, it is not available to the general users, and it becomes difficult for general users to convert to the local datum, i.e., modified Everest. In spite of these problems, fairly good maps were created integrating the data of GPS and satellite images, which could be used for developmental planning of the villages. Conclusion The comparative research study on various mapping technique carried out in the Riwazpur village reveals that mobile mapping through Pretek GPS is faster and more accurate. The mapping of small lanes in the village settlement areas is an advantage with this methodology since such details cannot be mapped even in the high-resolution data. Similarly the accuracy, cost and time for preparation of map using other technologies is often much higher and more time consuming and hence an attempt was made to use low cost mapping techniques, namely mobile mapping to prepare the rural settlement maps. This was further integrated with GIS for evolving strategies for better governance. However, the present method needs to be further refined for better accuracy in the maps and for integrating maps with the toposheets. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| © CSDMS. All rights reserved. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||