Current Situation of Open Source GIS

From: Open Source ChinaCommunityForum
Geographic Information SystemGeographic Information Systems (GIS) software relies on datasets covering the whole earth. To process a large amount of GIS data and its format, programmers have created several open-source libraries and GIS suites. This article briefly introduces the range and depth of GIS tools and libraries for UNIX and Linux.
GIS software was previously used only by geoologists and geoworkers. Since the Global Positioning System (GPS) service was provided online by Global Map and mobile phone companies, GIS software has become very popular. The GIS system can obtain and process spatial data describing the earth surface. Generally, after the spatial processing software formats, processes, and displays geographical data, the data is converted to geographical information. Sonar, radar, cameras, and other observation platforms can obtain the data. GIS stores the data in a 3D database, formats it, and transmits it. Sometimes the so-called four-dimensional (3D + time) conversion can even be used to show the development of data over time.

Of course, we can edit all the GIS data and must edit it frequently. Well-trained GIS experts can process most of the received dataset formats. Many (if not all) data formats comply with open standards. all operations in the GIS space can be used for open-source applications running on Linux, Major Berkeley Software Design (BSD), and some unix variants.Program. Most importantly, the GIS software suite can also run on Mac OS X.

Quantum GIS: open-source GIS data viewer
In recent years, GIS applications running on Linux have rapidly increased. In 1980s, programmers developed the Geographic Resources Analysis Support System (grass ). After modification in the late 1990s S, anyone with GIS knowledge and some Linux expertise can use grass to run a complete GIS system from the Linux Command Line or graphical user interface (GUI. Unfortunately, grass is extremely complex, causing inconvenience for beginners Running GIS on Linux. The GUI displays a large number of functions and command line identifiers. The target user is a GIS expert, rather than a newbie who attempts to create a network map.

In May 2002, developers developed gpled quantum GIS (qgis). This project is intended for beginners and intermediate users who need to access, display, and possibly edit GIS datasets. GIS users can deploy qgis as a separate GIS data viewer and editor, or as part of the GIS tool chain. The GIS tool chain can include qgis, grass software suite, a three-dimensional postgis database, and a map server that provides datasets and maps to users who access map websites over the Internet.

Qgis developers decided to use C ++-based QT toolkit to build the qgis interface, which is different from the main solution in previous practices (programmers mainly use Tcl/tk to create grass, tcl/TK was established in the late 1980s S ). Although qgis was developed based on the gpled QT toolkit, it can run on most Linux and UNIX variants, Microsoft Windows and Mac OS X.

Raster Data
Applications use two different data structures to store GIS data: raster data and vector data. You can add a three-dimensional data format for database storage, which is optimized for PostgreSQL processing-that is, the postgis data format. We classify postgis data as vector data for the following reasons.

Qgis processes all three types of data-raster data, vector data, and database. This is also an important direction for programmers who compile GIS data programming libraries. By imagining square grids or hexagonal units, you can easily visualize the data structure (in practice, the application uses square grids in most cases ). These grids cover a geographical area like a matrix and use a mathematical representation in a field named map algebra. GIS experts can add data to each grid, such as precipitation values or economic data, but it is difficult to describe complex irregular geographic shapes. The software usually relies on the similarity and the position of the grid's median to classify features (such as streets or coast), rather than based on feature descriptions encapsulated in metadata. Another possible interpretation relies on the use of color values associated with individual grids to classify grid groups into certain features.

The grid is arranged by row or column, which reflects the way the hard disk stores data. Many raster-based formats have basic image formats: the common raster-based image formats are bitmap (.bmp ). The Tagged Image File Format (. Tiff) is another commonly used raster-based image format. GIS Experts rename it as geotiff to meet the needs of geographic scholars. Raster-based data formats are more similar to images. Its accuracy depends on the number of grids that describe as few features as possible.

In the Raster Data Model, the accuracy of a map depends on the ratio of the map. Therefore, the resolution and accuracy of a map depend on the actual area represented by each grid. This data model is relatively simple and helps to model the data captured by GPS devices and satellite imaging. Some data formats are suitable for grid models. For example, the digital elevation model (DEM) data points are evenly arranged in a grid image. Decode elevation data in dem format to create a high-definition terrain. A few years ago, the US Geological Survey (USGS) released a very popular global DEM dataset to the public sector.

Vector Data
The emergence of Vector-based data formats has changed the lives of GIS experts. The new data viewer and editor, such as Thuban and qgis, do not have to bother to express Complex Vector-based formats, because it was built into the ability to edit and add vector-based data layers at the beginning. Grass has been around for more than 20 years, but it has only recently been available.

In short, vector data uses the simplest topological entity-points, lines, and polygon, and fix them in a two-dimensional Cartesian coordinate system to describe geographical features. The connection line is called an arc, and the point in the Cartesian coordinate system is called a node. The data structure is associated with the graph, and the graph theory is used as the mathematical basis. The so-called arc node list contains the arc and node. The List defines polygon and can stack with each other to indicate datasets in the opposite direction but that describe the same geographical region, thus forming a detailed map of data.

Qgis and other simple data viewer are used to detect various data formats and cover all datasets of this planetary system. Unlike some commercial products and even grass, you can easily install these viewers and use them on almost all major operating systems. The mileage may be slightly different, but it can always be successful.

Data format
Qgis supports many vector data formats, such as shapefile, MapInfo layer, and ArcInfo coverage. Vector Data requires much less storage than raster data because the arc node list simplifies and reduces the data required to represent the features contained in the map. This data is also very easy to search for maps or various layers represented by vectors. In early 1990s, after the commercial software suite ArcInfo GIS entered the market, it promoted the use of shapefile. Later, other files and data formats emerged, but there was no free open-source programming community. These achievements were still confined to the theoretical research and military planning of geographic scholars.

Grass
Grass 6.01 is recently released, which supports about 40 data formats. It breaks through the two-dimensional raster format, including the body element (voxel) or three-dimensional raster format. A large number of imaging and imaging modules greatly facilitate GIS experts to analyze data in a new way. This makes it possible to simulate and complete map production for a long time.

However, users still need to solve the problem because this version is intended for UNIX and Linux experts, its interface is rather messy and the installation process is also complicated. Further, the cause of this complexity is that grass provides a large number of libraries and tools. Fortunately, many grass documents have been written by students at various GIS technical levels, so Linux and UNIX beginners can deal with the complex requirements of grass installation.

Gdal and OGR
When talking about GIS data formats, it is inevitable that a large number of formats are to be supported, so that each data viewer and GIS application can be widely used. Open source GIS applications must include most open data format standards from ArcInfo to X Window systems. For example, the format of geotiff is a common example of open standards that are generally supported by most applications.

In the open source scope, grass, qgis, Thuban, and many other GIS applications use the basic library, geospatial data processing action Library (gdal ). Gdal is written in C and C ++ and can only contain one grid format. Another library is called OGR Simple Features Library (formerly called OpenGIS Simple Features reference implementation). Although it is built to adapt to the vector format existing in the gdal source tree, OGR depends on gdal. In fact, without the open-source license of gdal, most modern geospatial data viewers will be incredible. This library provides programmers with a common data model, including all raster data formats and vector data formats (through OGR ). Gdal also allows programmers to design raster data on world geographic coordinates (that is, geographic reference coordinates.

Postgis and OpenGIS
Without a sound data storage mechanism, GIS data in the public domain cannot exist. However, storage may not be as important as raster data, and its space structure may contain relatively simple digital data. Programmers only need to know some high-level structures, but most Programmers think that raster data is complex and occupies a large amount of storage.

OpenGIS solves these problems by generating vector data. Vector Data also refers to geometric objects, such as points, lines, polygon, and their combinations, it can be accessed in PostgreSQL that enables 3D databases (PostgreSQL's OpenGIS Implementation standard is postgis ). GIS data stored in PostgreSQL databases can be searched using SQL-92.

Currently, programmers can access the entire open source GIS application, which is largely dependent on UNIX and Linux systems. These results benefit from common open standards, and most Internet map production fields tend to rely on these standards. All programmers who process geographic data in any form will encounter the same basic library problems as Linux programmers on glibc. GIS programmers cannot determine which data formats and basic libraries to use, even if they just want to use the keyboard to write a data filter script or delete the tool chain.

Conclusion
When talking about open-source and global environment phenomena, it is difficult to think of geographic data sets and GIS applications. However, open-source applications such as grass and qgis are trying to make public domain GIS datasets available for programmers and technical users so that they do not have to use commercial products. Databases such as gdal and OGR can process GIS data on a general open source basis without affecting the integrity of Open GIS data standards.