GIS are modern mobile geoinformation systems that have the ability to display their location on a map. This important property is based on the use of two technologies: geoinformation and global positioning. If the mobile device has a built-in GPS receiver, then with the help of such a device it is possible to determine its location and, consequently, the exact coordinates of the GIS itself. Unfortunately, geoinformation technologies and systems in the Russian-language scientific literature are represented by a small number of publications, as a result of which there is almost no information about the algorithms underlying their functionality.
GIS classification
The division of geographic information systems occurs according to the territorial principle:
- Global GIS has been used to prevent man-made and natural disasters since 1997. Thanks to these data, it is possible for relativelypredict the scale of the disaster in a short time, draw up a plan for the aftermath, assess the damage and loss of life, and organize humanitarian actions.
- Regional geoinformation system developed at the municipal level. It allows local authorities to predict the development of a particular region. This system reflects almost all important areas, such as investment, property, navigation and information, legal, etc. It is also worth noting that thanks to the use of these technologies, it became possible to act as a guarantor of the life safety of the entire population. The regional geographic information system is currently being used quite effectively, helping to attract investment and the rapid growth of the region's economy.
Each of the above groups has certain subtypes:
- The global GIS includes national and subcontinental systems, usually with state status.
- To the regional - local, subregional, local.
Information about these information systems can be found in special sections of the network, which are called geoportals. They are placed in the public domain for review without any restrictions.
Working principle
Geographic information systems work on the principle of compiling and developing an algorithm. It is he who allows you to display the movement of an object on a GIS map, including the movement of a mobile device within the local system. Toto depict this point on the terrain drawing, you need to know at least two coordinates - X and Y. When displaying the movement of an object on a map, you will need to determine the sequence of coordinates (Xk and Yk). Their indicators should correspond to different points in time of the local GIS system. This is the basis for determining the location of the object.
This sequence of coordinates can be extracted from a standard NMEA file of a GPS receiver that has performed real movement on the ground. Thus, the algorithm considered here is based on the use of NMEA file data with the coordinates of the object's trajectory over a certain territory. The necessary data can also be obtained as a result of modeling the motion process based on computer experiments.
GIS algorithms
Geoinformation systems are built on the initial data that is taken to develop the algorithm. As a rule, this is a set of coordinates (Xk and Yk) corresponding to some object trajectory in the form of an NMEA file and a digital GIS map for a selected area. The task is to develop an algorithm that displays the movement of a point object. In the course of this work, three algorithms were analyzed that underlie the solution of the problem.
- The first GIS algorithm is the analysis of NMEA file data to extract from it a sequence of coordinates (Xk and Yk),
- The second algorithm is used to calculate the track angle of the object, while the parameter is counted from the direction toeast.
- The third algorithm is for determining the course of an object relative to the cardinal points.
Generalized algorithm: general concept
The generalized algorithm for displaying the movement of a point object on a GIS map includes the three previously mentioned algorithms:
- NMEA data analysis;
- calculation of the object's track angle;
- determining the course of an object relative to countries around the globe.
Geographical information systems with a generalized algorithm are equipped with the main control element – a timer. Its standard task is that it allows the program to generate events at certain intervals. Using such an object, you can set the required period for the execution of a set of procedures or functions. For example, for a repeatable countdown of a one-second time interval, you need to set the following timer properties:
- Timer. Interval=1000;
- Timer. Enabled=True.
As a result, the procedure for reading the X, Y coordinates of the object from the NMEA file will be launched every second, as a result of which this point with the received coordinates is displayed on the GIS map.
The principle of the timer
The use of geographic information systems is as follows:
- Three points are marked on the digital map (symbol - 1, 2, 3), which correspond to the trajectory of the object at different momentstime tk2, tk1, tk. They are necessarily connected by a solid line.
- Enabling and disabling the timer that controls the display of the object's movement on the map is carried out using the buttons pressed by the user. Their meaning and a certain combination can be studied according to the scheme.
NMEA file
Let's briefly describe the composition of the GIS NMEA file. This is a document written in ASCII format. In essence, it is a protocol for exchanging information between a GPS receiver and other devices, such as a PC or PDA. Each NMEA message begins with a $ sign, followed by a two-character device designation (GP for a GPS receiver) and ends with \r\n, a carriage return and line feed character. The accuracy of the data in the notification depends on the type of message. All information is contained in one line, with fields separated by commas.
In order to understand how geographic information systems work, it is enough to study the widely used $GPRMC type message, which contains a minimal but basic set of data: the location of an object, its speed and time.
Let's consider a certain example, what information is encoded in it:
- date of determination of the coordinates of the object - January 7, 2015;
- Universal time UTC coordinates - 10h 54m 52s;
- object coordinates - 55°22.4271' N and 36°44.1610' E
We emphasize that the coordinates of the objectare presented in degrees and minutes, with the latter being given with an accuracy of four decimal places (or a dot as a separator between the integer and fractional parts of a real number in USA format). In the future, you will need that in the NMEA file, the latitude of the location of the object is in the position after the third comma, and the longitude is after the fifth. At the end of the message, the checksum is transmitted after the '' character as two hexadecimal digits - 6C.
Geoinformation systems: examples of compiling an algorithm
Let's consider an NMEA file analysis algorithm to extract a set of coordinates (X and Yk) corresponding to the object's motion trajectory. It is made up of several successive steps.
Determining the Y coordinate of an object
NMEA data analysis algorithm
Step 1. Read GPRMC string from NMEA file.
Step 2. Find the position of the third comma in the string (q).
Step 3. Find the position of the fourth comma in the string (r).
Step 4. Find the decimal point character (t) starting from position q.
Step 5. Extract one the character from the string at position (r+1).
Step 6. If this character is equal to W, then the NorthernHemisphere variable is set to 1, otherwise -1.
Step 7. Extract (r- +2) characters of the string starting at position (t-2).
Step 8. Extract (t-q-3) characters of the string starting at position (q+1).
Step 9. Convert strings to real numbers and calculate the Y coordinate of the object in radian measure.
Determining the X coordinate of an object
Step 10. Find the position of the fifthcomma in string (n).
Step 11. Find the position of the sixth comma in string (m).
Step 12. Starting from position n, find the decimal point symbol (p). Step 13. Extract one character from the string at position (m+1).
Step 14. If this character is equal to 'E', then the EasternHemisphere variable is set to 1, otherwise -1. Step 15. Extract (m-p+2) characters of the string starting at position (p-2).
Step 16. Extract (p-n+2) characters of the string starting at position (n+ 1).
Step 17. Convert the strings to real numbers and calculate the X coordinate of the object in radian measure.
Step 18. If the NMEA file is not read to the end, then go to step 1, otherwise go to step 19.
Step 19. Finish the algorithm.
Steps 6 and 16 of this algorithm use the NorthernHemisphere and EasternHemisphere variables to numerically encode the object's location on Earth. In the northern (southern) hemisphere, the variable NorthernHemisphere takes the value 1 (-1), respectively, similarly in the eastern (western) hemisphere EasternHemisphere - 1 (-1).
GIS application
The use of geographic information systems is widespread in many areas:
- geology and cartography;
- trade and services;
- inventory;
- economics and management;
- defense;
- engineering;
- education, etc.
