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PDF | With this third edition of Open Source GIS: A GRASS GIS Approach, we enter the new era of GRASS6, the first release that includes substantial new code .
Table of contents
- A GRASS GIS Approach
- Open Source GIS, A GRASS GIS Approach - Dimensions
- Open Source GIS: A GRASS GIS Approach. Third Edition.
- Open Source GIS: A GRASS GIS Approach
- ITC - University of Twente
Also multiple-return lidar data are included. The examples throughout this book are based on this data set. Furthermore, new derivative maps are generated and explained. After a brief introduction to GIS concepts, map projections and coordinate systems are explained. The fourth chapter describes the properties of GRASS raster and vector data and provides ex- tensive information on import and export of a wide range of data formats.
Management, display, analysis and modeling using raster and vector data is covered in the next two chapters, again using hands-on examples based on the sample data set. An extensive chapter is devoted to the satellite image processing and analysis as a special case of raster data application. The Appendix provides equations used in some of the modules. References to lit- erature provide access to detailed information about the given topic. We use the following conventions throughout the book. Commands which you can type in are written in typewriter font, for example: r.
Wherever [ Lines starting with symbol indicate comments that are not executed by the shell. This character is usually not necessary when typing, we often used it here for formatting reasons. Otherwise the subsequent line s are ignored. It is therefore useful to verify the most recent command usage in the related manual page.
Each thematic layer can be stored using an appropriate data model depending on the source of data and their potential use. The spatial component can be represented using one of the two basic approaches Figure 2. Similarly, a city can be represented as a point or as an area.
A GRASS GIS Approach
Note that we use the terms small and large scale in the cartographic sense, for example, million is small scale, is large scale. Raster data model Raster is a regular matrix of values Figure 2. If the values are assigned to grid cells area units, pixels , it represents an image satellite image, scanned map, converted vector map. If the cell values represent category numbers, one or more attributes can be assigned to that cell using a database. For example, a soil type with category number 3 can have attributes describing its texture, acidity, color and other properties.
The grid cells are organized and accessed by rows and columns. The area represented by a square grid cell is computed from the length of its side, called resolution. Resolution controls the level of spatial detail captured by the raster data.
Most data are represented by a 2D raster, with the grid cell unit area called a pixel; volume data can be stored as a 3D raster with a unit volume called a voxel volume pixel. General d-dimensional raster formats are used for spatio-temporal or multispectral data e. HDF format1. However, it can also be used for data usually represented by lines and polygons such as roads or soil properties, especially for scanned maps.
The raster data model was designed with a focus on analysis, modeling and image processing. GRASS has extensive support for the raster data model. Vector data model Vector data model is used to represent areas, lines and points Figure 2. The vector data model is based on arc-node representation, consisting of non-intersecting lines called arcs. An arc is stored as a series of points given by x, y or x, y, z coordinate pairs or triplets with height.
The two endpoints of an arc are called nodes. Points along a line are called vertices. The arcs form higher level map features: lines e.
Arcs that outline areas polygons are called area edges or boundaries. A complete area description includes a centroid. A 3D volume is a closed set of faces including a 3D centroid kernel. Not all GIS software packages support 3D vector data types. In addition to the coordinate information, the vector data model often includes information about the data topology which describes the relative po- sition of objects to each other see Section 6.
Each map feature is assigned a category number which is used to link the geometric data with descriptive, attribute data such as category labels or multiple attributes stored in a database. Point features e. A value or a set of attributes numerical or text is assigned to each point. Continuous spatial data can be represented by vector data model using isolines, point clouds or various types of irregular meshes; however, such representations usually lead to more complex algo- rithms for analysis and modeling than the raster data model.
Data dimensions in a Geographical Information System after Rase, Attributes — GIS and databases Attributes are descriptive data provid- ing information associated with the geometrical data.
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Multiple attributes can be stored and managed for each vector object. One or several attribute tables can be linked to a vector map. GIS usually includes tools for transfor- mation between the vector and raster data model. For example, elevation can be measured as vector point data, then interpolated into a raster map which is then used to derive contour lines as vector data.
Dimensions of geospatial data In general, Earth and its features are located and evolve in 3D space and time.
Open Source GIS, A GRASS GIS Approach - Dimensions
Elevation as a third dimension is usually stored as a separate raster map representing a surface within three-dimensional space often referred to, not quite correctly, as a 2. Elevation can be also added as a z-coordinate or as an attribute to vector data.
Three-dimensional data can change in time, adding the fourth dimension. GIS provides the most comprehensive support for 2D data. GeoServer, MapQuest , or as a subsystem of a larger software package supporting han- dling of geospatial data needed in its applications e. The multipurpose systems are often built from smaller components or mod- ules which can be used independently in application oriented systems. The multipurpose GIS usually stores the georeferenced data as thematic maps.
Each geographic feature or theme, such as streams, roads, vegetation, or cities is stored in a separate map using the vector or raster data model. The data sets are stored on central server s and users access the data as well as the display and analysis tools through the Internet. Almost every multipurpose GIS software includes tools supporting development of Web-based applications. While creating digital and hardcopy maps has been the core GIS function over the past decade, the emphasis is shifting towards Web Services, spatial analysis and modeling.
OSGeo foundation plays a major role in the development of these new tech- nologies. GIS functionality is used to solve spatial problems in almost every area of our lives. Here are a few examples. Geodetic or map datum A set of constants specifying the coordinate sys- tem used for calculating the coordinates of points on Earth is called a geodetic datum. For maps to match, their coordinates must be computed using the same datum. Direct projec- tion of a spherical object to a plane cannot be performed without distortion.
Open Source GIS: A GRASS GIS Approach. Third Edition.
The most common approach is to project the spheroid onto a developable surface, such as a cylinder or a cone that can be developed into a plane with- out deformation tearing or stretching , see Figure 2. The conformal projection preserves angles shapes for small areas and is often used for navigation and national grid systems. The equidistant projection preserves certain relative distances and is used for measurement of length. The equivalent projection preserves area and is used for measurement of areas. Each of the properties angle, dis- tance, area is preserved at the expense of the others.
The map projection is usually selected depending on the application because there is no perfect solution to the projection problem. Most coordinate systems used for land surface mapping use conformal projections. Add to Basket. Book Description Springer.
Open Source GIS: A GRASS GIS Approach
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ITC - University of Twente
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