GIS are modern geographic information mobile systems that have the ability to display their location on a map. The basis of this important property is the use of two technologies: geographic information and global positioning. If the mobile device has a built-in GPS receiver, then with the help of such a device you can determine its location and, therefore, 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 this there is almost no information about the algorithms that underlie 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 to predict the magnitude of the disaster in a relatively short time, draw up a plan to eliminate the consequences, assess the damage caused and human losses, as well as organize humanitarian actions.
- Regional geographic information 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, for example, 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, contributing to attracting investment and the rapid growth of the region's economy.
Each of the above groups has certain subspecies:
- The global GIS includes national and subcontinental systems, usually with state status.
- In 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.
Principle of operation
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. In order to 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 the 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 an object.
This sequence of coordinates can be extracted from the standard NMEA file of the GPS receiver that performed the actual movement on the ground. Thus, the basis of the algorithm considered here is the use of NMEA file data with the coordinates of the object's trajectory over a specific territory. The necessary data can also be obtained as a result of modeling the motion process based on computer experiments.
GIS Algorithms
Geographic information systems are built on the source data, which are taken to develop the algorithm. As a rule, this is a set of coordinates (Xk and Yk) corresponding to a certain trajectory of the object in the form of an NMEA file and a digital GIS map in a selected area. The task is to develop an algorithm that displays the motion of a point object. In the course of this work, three algorithms underlying the solution of the problem were analyzed.
- The first GIS algorithm is the analysis of NMEA file data in order to extract a sequence of coordinates (Xk and Yk) from it,
- The second algorithm is used to calculate the path angle of the object, while the parameter is counted from the east direction.
- The third algorithm is to determine the course of an object relative to the countries of the world.
Generalized Algorithm: General Concept
The generalized algorithm for displaying the motion of a point object on a GIS map includes the three algorithms indicated above:
- NMEA data analysis;
- calculation of the path angle of the object;
- determining the course of an object relative to countries of the entire globe.
Geographic information systems with a generalized algorithm are equipped with the main control element - a timer (Timer). Its standard task is that it allows the program to generate events at certain intervals. Using this object, you can set the required period for performing a set of procedures or functions. For example, for a repeatedly executed time interval countdown of one second, the following timer properties must be set:
- Timer.Interval = 1000;
- Timer.Enabled = True.
As a result, every second, the procedure of reading the X, Y coordinates of the object from the NMEA file will be launched, as a result of which this point with the obtained 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 times tk2, tk1, tk. They are necessarily connected by a solid line.
- Turning on and off the timer that controls the display of the movement of an object on the map is carried out using buttons that are pressed by the user. Their meaning and a certain combination can be studied according to the scheme.
NMEA file
Let us briefly describe the composition of the NMEA GIS file. This is an ASCII document. In fact, it is a protocol for exchanging information between a GPS receiver and other devices, such as a PC or PDA. Each NMEA message starts with a $ sign, followed by a two-character device designation (for the GPS receiver - GP) and ends with the sequence \ r \ n - a carriage return and a new line. The accuracy of the data in the notification depends on the type of message. All information is contained in one line, and the fields are separated by commas.
In order to understand how geographic information systems work, it is quite enough to study a widely used message such as $ GPRMC, which contains a minimal but basic data set: the location of the object, its speed and time.
Consider, using a specific example, what information is encoded in it:
- Date of determining the coordinates of the object - January 7, 2015;
- UTC world time for determining coordinates - 10h 54m 52s;
- the coordinates of the object - 55 ° 22.4271 'N and 36 ° 44.1610 'E
We emphasize that the coordinates of the object are presented in degrees and minutes, and the last indicator is given up to four decimal places (or a point as a separator of the integer and fractional parts of a real number in the USA format). In the future, you will need the fact that in the NMEA file the latitude of the object’s location is located after the third comma, and longitude - after the fifth. At the end of the message, a checksum is transmitted after the '*' character in the form of two hexadecimal digits - 6C.
Geographic Information Systems: Examples of Algorithm Design
Consider an algorithm for analyzing an NMEA file to extract a set of coordinates (X and Yk) corresponding to the object’s trajectory . It is composed of several successive steps.
Determining the Y coordinate of an object
NMEA Data Analysis Algorithm
Step 1. Read the GPRMC line from the NMEA file.
Step 2. Find the position of the third comma in the line (q).
Step 3. Find the position of the fourth comma in line (r).
Step 4. Find, starting at position q, the decimal point character (t).
Step 5. Extract one character from the string at position (r + 1).
Step 6. If this character is equal to W, then the NorthernHemisphere variable gets the value 1, otherwise -1.
Step 7. Extract (r— + 2) characters of the string, starting at position (t-2).
Step 8. Extract (tq-3) characters from 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 fifth comma in line (n).
Step 11. Find the position of the sixth comma in line (m).
Step 12. Find, starting at position n, the decimal point character (p).
Step 13. Extract one character from the string at position (m + 1).
Step 14. If this character is 'E', then the EasternHemisphere variable gets the value 1, otherwise -1.
Step 15. Extract (m-p + 2) characters from the string starting at position (p-2).
Step 16. Extract (p-n + 2) characters from the string starting at position (n + 1).
Step 17. Convert 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, go to step 1, otherwise go to step 19.
Step 19. Finish the algorithm.
In steps 6 and 16 of this algorithm, the NorthernHemisphere and EasternHemisphere variables are used to numerically encode the location of the object on Earth. In the northern (southern) hemisphere, the NorthernHemisphere variable 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;
- cadastre;
- economics and management;
- defense;
- engineering;
- education, etc.