CARTOGRAPHY

 

Cartography is an art, science and technology of creating maps together with their study as scientific documents. Maps are graphic representation of geographical environment. Map-making process is a series of information transformation, each of which has the power to alter the appearance of final product. The fundamental function of a map is to bring things into view. All maps are concerned with two elements of reality namely location and attribute. From these two basic elements, any relationship can be formed which depends on the purpose of map.

Creating a map depends largely on its intended use. Depending upon the purpose, maps can be classified into general reference maps; and thematic maps. General reference maps show the location of a variety of different features like roads, rivers, political boundaries etc. Thematic maps concentrate on the distribution of a single and multiple attributes or the relationship among several attributes. Thematic maps tell about a theme. A simple example for a thematic map is a map used for weather forecasting.

            Traditional surveying methods, which were used for cartography, are laborious and time consuming. Also the accuracy of traditional methods is low. Cartography is in the midst of a revolution in technology. This revolution is referred to as digital cartography. It is caused by widespread use of electronics and computers in the field of cartography. Increasing use of cartographic products is another factor for this revolution. This paper is an attempt to explain the new trends in cartography. This paper also deals with a brief history of cartography.

The oldest known maps are preserved on Babylonian clay tablets, which dates back to 2300 B.C. The concept of a spherical Earth was well known among Greek philosophers at the time of Aristotle (350 B.C.) and has been accepted by all geographers since.

The oldest known maps are preserved on Babylonian clay tablets, which dates back to 2300 B.C. The concept of a spherical Earth was well known among Greek philosophers at the time of Aristotle (350 B.C.) and has been accepted by all geographers since.

Babylonians were the first to use sexagesimal system. Around 1320 B.C., Egyptians developed accurate system of land surveying. Greeks developed the foundation of modern cartography. They were first to develop concept of latitude and longitude. Around 400 B.C., Romans developed many military maps based on flat-earth concept. Around 1300, the use of magnetic compass in cartography was brought to western world from china. There was an end to speculative mapping from 1650 onwards. Cartographers began to explore the power of electronics in early 1950’s.

When data collection is discussed in cartography, an important term comes into picture “Ground control points”. Ground control points are fixed points, which are used to determine two or three-dimensional positions of an object. This paper deals with data collection by satellite remote sensing method.

           

After data has been collected the actual cartographic process starts. The data will be fed into the computer database and analysis of data and design of maps are done. After this the map is scaled according to the requirements and a suitable map projection is chosen to view the map on the computer or a hardcopy can be obtained.  Choosing a map projection is very important because the resulting map should have minimum distortion.

SATELLITE REMOTE SENSING           

                        Remote sensing is the process for collecting, storing, and extracting environmental information from images of the ground acquired by devices not in direct physical contact with the features being studied. Remote sensing devices detect and record radiant energy. Remote sensing of land surface features is based on detection of radiant energy called electromagnetic radiation. Visible light is an example of radiant energy.

            All objects with temperatures above absolute zero continuously emit a broad range of electromagnetic wavelengths and frequencies. The range of electromagnetic wavelengths and frequencies can be termed as electromagnetic spectrum. The electromagnetic spectrum has been divided into energy regions. It’s as shown in the figure below. Remote sensing devises operate in all energy regions except radio, x-ray and gamma ray.

Remote sensing instruments detect and record the energy of photon in the band or bands to which the sensors is sensitive. The amount of energy detected depends on the inherent energy of photons in the band and on the number of photons reaching the detector during the short time interval of energy collection. The number of photons reaching the detector varies according to the amount of energy emitted by the illumination source, the amount of energy absorbed by the atmosphere and the degree to which ground objects reflect and emit energy.

            The portions of the electromagnetic appropriate for remote sensing are restricted by atmospheric absorption. This atmospheric absorption reduces the percentage of energy transmitted through the atmosphere to the ground and back through the atmosphere to the sensing instrument. Portion of low absorption, called atmospheric windows, are the portions of the spectrum used for remote sensing using multispectral scanners. Spectral signatures of objects are identified and mapped by remote sensing methods. The figure below shows the atmospheric window where remote sensing is possible.

            Satellite is used as a platform for remote sensing devices. All the satellites used for remote sensing are sun synchronous satellites. Indians are considered pioneers in remote sensing space technology. Indian space research organization (ISRO) has launched IRS satellites; the data received by these are used for various purposes in India and abroad. For example IRC-P5 (CARTOSAT-1) caters to the needs of cartographers and terrain modelling applications. The satellite will provide cadastral level information up to 1:5000 scale and will be useful for making 2-5m contour maps.

            Images created by a variety of remote sensing devices are used in map compilation and updating. Remote sensing images are also geometrically rectified and mosaicked to form planimetrically correct photomaps.  Energy can be recorded photographically and electronically, and the record may be in hardcopy or digital data file format.

            Cartographers should understand the basic operation of major remote sensing instruments and be well acquainted with the forms of imagery each creates. 

Photographic imaging

            Aerial cameras, which were first developed during World War I, have long provided aerial photographs for topographic map compilation and resource inventories. The standard mapping camera produces individual large-format 9x9inch. High precision 6-inch or 12-inch focal length lenses can be used to minimize spatial distortions on the photo. The geometric accuracy of aerial photographs depends on the camera and lens design. Photo appearance and resolving power depend primarily on the type of film used.

Photographic films are composed of a transparent plastic base coated on one side with an emulsion of silver salt crystals. These crystals are sensitive to electromagnetic energy. The emulsion is exposed to photons reflected from ground objects and focused by the camera lens. Sliver crystals struck by a sufficient number of photons change their chemical properties so as to turn black upon film development. Other crystals exposed below this energy level remain in their original state. Photographic emulsion can record energy between 0.3 and 1.2mm or from the near-UV through the near-IR. Four emulsions – panchromatic, black and white infrared, true-colour, and colour-infrared have been developed for remote sensing within this spectral range.

Electronic imaging

Photographic film emulsion used in remote sensing can only be made sensitive to energy in the 0.3-1.2mm range. In all other regions of the electromagnetic spectrum, we must use electronic detection and recording instruments.

The three forms of imaging instruments currently in use are termed matrix array, pushbroom, and whiskbroom. These terms roughly describe the way these instruments create images. The basic principle of working is the same. Electronic sensors measures the intensity of radiation striking its surface, converts the intensity to a digital number, and transmits this number to a large hard disk in the control computer.

Thermal scanners are whiskbroom instruments that detect middle-IR energy in the 3-14mm range. The detector is sensitive to the temperature of the scanning device as well as to energy from the ground. Therefore, the entire instrument must be cooled with liquid nitrogen to an extremely low temperature and enclosed in a heatproof box. Since thermal scanners do not operate in the visible region, they may be used to record the nearly equal mixture of reflected and emitted thermal IR energy during the day, or emitted energy alone at night. These are used to detect forest fires, fresh lava, discharge form power plants, soil moisture differences etc.

            Multi spectral scanners simultaneously record electromagnetic energy in several narrow spectral bands. Most multi spectral scanners are similar in design to the whiskbroom scanner. These are used in study of crops, natural vegetation types, soil moisture variations and land use.

            Passive microwave sensors detect and record energy in the 0.1-25 cm band of the microwave region. Passive microwave sensors are similar to whiskbroom scanners except that detection of microwave energy requires an antenna device called microwave radiometer. Passive microwave images are used to map soil moisture, ocean surface conditions, inventorying the water content of snow fields, delineating the location of ice-water boundaries, and distinguishing among geologic strata.

IMAGE PROCESSING

            Image processing refers to manual and digital techniques used to improve geometry, enhance image appearance, identify feature in an image, and extract selected information from an image. Cartographers use digital image processing systems to geometrically rectify images, prepare contrast enhanced multicolour image maps, create thematic maps by extracting information from image data, and simplify such thematic maps. Before this the image should be in raster format. In raster format each image is identified by a matrix of pixels.

            Original image must be rectified to match a particular map projection surface or grid coordinate system laid over the projection surface. Mathematical equations are used for linear coordinate transformation. Image rectification involves placing satellite image on to a base map so that the coordinate system can be used for the satellite image.

            Image enhancement is done to improve the appearance of image data displayed. Image contrast can be improved by using spatial filter, density slicing, and image combination.

THEMATIC INFORMATION EXTRACTION

            The extraction of thematic information from digital image data is a very important image processing activity. There are two general approaches to compiling thematic maps through image processing namely manual image classification and digital image classification.

            Digital image classification involves use of image processing software, which classify each pixel, based on the reflectance value in each spectral band. Cartographers perform two major forms of digital image classification namely supervised and unsupervised classification. Supervised classification involves determination of the digital reflectance characteristic of pixels known to be of each feature class. Unsupervised classification is based on finding inherent clusters of similar reflectance values. Digital image simplification can be done according to the requirements. In this way enhance image maps can be produced.

APPLICATIONS OF REMOTE SENSING

·         Used to create digital terrain models, which are used to study topographical details.

·         Used to study soil and rock types.

·         Used in highway design.

·         Digital maps created by remote sensing method can be used with GIS for studying

the variation of an attribute or to find relationships among them.

·         Integration of thematic maps through GIS for identification of ground water potential zones.

·         Used for weather forecasting.