Raster data
In addition to vector data, raster data are spatial computer-readable data ( geodata ) with pictorial information content. The linking of geodata and factual data creates geospatial information , i. H. interpreted spatial data relating to locations or areas on the earth's surface. Raster data describe the object geometry through digital images, such as aerial and satellite photographs, but also scanned maps and thus open up new perspectives.
definition
If geodata are based on the properties of the raster data model, they are represented by a matrix , i.e. H. through a grid of horizontal rows and vertical columns (also called grid ) as a set of picture elements ( pixels ) or cells. The cells are homogeneous in size within the matrix, the size depending on the resolution . The individual cells are each assigned different values, which describe the space shown in the cell, e.g. B. temperatures, altitudes or pollution. As a semantic statement , the values then determine which grayscale or color (uniform area filling) the respective cell takes. The larger and finer the resolution, the more detailed a geo-object is described by values. When capturing raster data, a basic distinction is made between primary recording systems, which address image recording directly in raster form, and secondary recording systems, which can convert any analogue originals (paper, film, etc.) into digital raster data.
application
Raster data began in the 1960s with the analysis of space images. Today, with the high level of development of raster technology , continuous, geographical phenomena occurring over the entire surface of the earth are suitable for representation as a raster model. This can be, for example, the distribution of a pollutant concentration. Raster data are created on the basis of samples. Support points obtained through measurement can be regular or irregular. The example mentioned of the pollutant concentration has irregular bases. Another example would be the terrain height above sea level. The surface is generated by different interpolation methods . Spatially close points have similar values (auto-correlation). In addition, the modeling of fuzzy phenomena is one of their strengths. Almost all input-output devices (mouse, scanner, digital camera) are based on raster data. This is because modern computers have sufficient power to process more memory-intensive raster graphics . This enables both digital processing of image recordings and of analog / digitally converted graphic information in raster form without any loss of quality or resolution.
Structure of raster data
Raster data is based on areas. The study area is broken down into small, mainly regular (mostly, but not necessarily square) cells. The regular cell division is always based on simple geometric figures such as squares, rectangles, triangles or hexagons. Only in exceptional cases does cell division occur irregularly. The grid cells store the respective characteristics of this system. These are also called grid cells, grid cells, grid meshes, pixels or picture elements. The numerical values of the individual pixels can have a purely quantitative or qualitative (object key-coded) meaning. The raster data set thus contains information about position, orientation, raster size and type of numerical value coding and thus allows a clear thematic and geometric pixel assignment.
Storage of raster data
The storage of raster data d (x, y) always takes place line by line. The grid elements or picture elements generally have a constant record length (M). A numerical value (N) is assigned to each pixel. This “gray value” represents reflected or emitted radiation values that were recorded in a specific spectral range .
File structure
A few parameters determine the geometric aspect of raster data. One example is the GRID format. The three determining parameters are the specification of the coordinates of the origin, the grid size and the number of rows and columns. A NoData value is also defined. These are used to identify the areas for which no values are available that result from the rectangular section. Since "0" can be a meaningful measured value, it should not be used as a NoData value. Negative numbers outside the valid range (-9999) are better suited.
File formats
Typical exchange and storage formats for raster data are:
- JPEG (Joint Photographic Expert Group) with a color depth of up to 24 bits (approx. 16.7 million different colors). The advantage lies in the relatively high degree of compression, so that relatively small amounts of data are created. The disadvantage is the associated relatively poor image quality.
- GIF (Graphics Interchange Format) with a color depth of up to 8 bits (approx. 256 different colors) and therefore more suitable for simple graphics. The advantage, however, lies in the lossless image compression. Further advantages lie in functions such as "interlacing" (parallel image transfer during the loading process), "animated GIFs" (image sequences through successive GIF images) and the option of defining transparency.
- TIFF (Tagged Image File Format) and FITS ,
- BMP (Windows Bitmap)
Image data
Image data is a special group of raster data. These are based on regular scanning of surfaces. This is done with the help of scanners or cameras. Aerial and satellite photographs can be named as examples . The recorded reflection values of the earth's surface are transferred to gray values of a raster data set. Each grid cell has a specific brightness value. The available gray values are calculated in the unit “ bit ”. This shows the radiometric resolution of a digital recording. A common resolution is 8 bit (Landsat ETM +). Geo-information is obtained from image data through geo-referencing and interpretation.
Problem areas
- The high memory requirements of the individual pixels in a graphic are problematic for further processing in geographic information systems .
- Limited scalability (enlarging or reducing the raster graphic ) after defining the pixels. The original topography is lost.
- Often unsuitable for analytical and cartographic processes because they do not contain any structural information (e.g. about the position and extent of objects).
Comparison of raster and vector data
The second fundamental model of spatial analysis methods is the vector model . There is a clear separation of factual and geometric data. Reproduced, thematic data layers have an associated table with attribute data. The advantages of the vector data are their good possibilities of a thematic object description, the good selectability of individual objects according to quality and name as well as their good data compression . Vector data are also older than raster data. They were already used in the early 1950s. In contrast to vector data, raster data require more storage space . However, they also have some advantages over the vector data. These include the simple database structure and no cumbersome geometric access options, as well as no time-consuming calculations of data links and geometric neighborhood relationships. A vector aggregates while the raster disaggregates. The two methods are therefore not in competition with each other, but have specific fields of application for which they are particularly suitable: The vector form is only suitable to a limited extent for thematic evaluations, but extremely well for pure storage, continuation and administration. With raster data, on the other hand, complex thematic evaluations can be implemented flexibly and particularly elegantly.
Individual evidence
- ↑ a b fe-lexikon.info
- ↑ a b c d W. Göpfert: Spatial Information Systems. Basics of the integrated processing of point, vector and raster data, applications in cartography, remote sensing and environmental planning. 2nd Edition. Wichmann, Karlsruhe 1991, ISBN 3-87907-232-9 , pp. X-Y.
- ↑ a b c d e S. Lang, T. Blaschke: Landscape analysis with GIS. Ulmer, Stuttgart 2007, ISBN 978-3-8252-8347-6 .
- ↑ a b c d R. Buchfelder: Introduction to GIS. The probe data model. Seminar paper. Regensburg 2004. ( PDF file ( page no longer available , search in web archives ) Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. ).
