USING OF GPS FOR DATA COLLECTING FOR GIS
Mgr. David Krèmáø
Comenius University of Natural Sciences
Department of Hydrogeology
Mlynska dolina, G-115
842 15 Bratislava, Slovakia
tel: +4217-60296565, fax: +4217-65428867
email: krcmar@fns.uniba.sk
KEY WORDS
GIS, GPS, mapping, data, program, measuring
ABSTRACT
According the definition - GIS is a tool for capturing, storing and manipulating of the spatially orientated data. It is clear from the definition that the data are one of the most important and most costly part of the GIS system. Sources of the data could be already processed data in analogue or digital form or direct results from the mapping activities. GPS as a modern tool is a very effective instrument for the mapping purposes. It is easy by the help of the modern apparatuses gain spatial information about mapping objects on the Earth with centimeter accuracy. A big advantage of GPS system is easy manipulation with apparatuses, every weather measuring and real time mapping possibilities. It is marked that the usage of GPS system is really wide and effective for broad scope of applications.
Introduction
The term Geographical Information System (GIS) used in North America is frequently applied to geographically oriented computer technology. A definition could be: GIS is a system for capturing, storing, checking, manipulating, analyzing and displaying data which are spatially referenced to the Earth (Maguire et. al., 1992). We know that data are the most important part of GIS. Data are derived from many different sources, both analogue and digital. Data can be gained also by conventional or GPS mapping.
Global positioning system (GPS) is based on constellation of 24 satellites orbiting the Earth at a very high altitude. The basic principles behind GPS are quite simple and can be described in five steps (Hurn, 1989):
Measurement methods
Measurement in the field with GPS apparatus is very simple and differs from used method. We can measure our absolute position in terms of longitude, latitude and ellipsoid height or our relative position (vectors). Important question is, how precise we can measure our position? In table no.1 you can find summary of error sources and position accuracy (Trimble homepage).
table no.1: Summary of GPS Error Sources
Typical Error in Meters (per satellite) |
||
Standard GPS |
Differential GPS |
|
Satellite Clocks |
1.5 |
0 |
Orbit Errors |
2.5 |
0 |
Ionosphere |
5.0 |
0.4 |
Troposphere |
0.5 |
0.2 |
Receiver Noise |
0.3 |
0.3 |
Multipath |
0.6 |
0.6 |
SA |
30 |
0 |
Typical Position Accuracy |
||
Horizontal |
50 |
1.3 |
Vertical |
78 |
2.0 |
3-D |
93 |
2.8 |
By absolute position we can go into submeter accuracy (up to 2 meters in differential GPS). In measuring vectors (relative position) we can make measurements down to a centimeter, even few millimeters accuracy (depending on measuring method). Types of measuring methods are listed in table no.2.
table no.2: Measuring methods by GPS
method |
precision (in millimeters +/- 1ppm*baseline) |
static |
1-2 |
fast static |
5-10 |
reoccupation |
5-10 |
stop & go |
10-20 |
kinematic with static initialization |
10-20 |
on the fly kinematic |
10-20 |
real time kinematic |
10-20 |
During mapping there are many important factors which should be considered. Time necessary for one point observation is important one and differs from used method. The static method is most time consuming, but on the other hand it is also the most precise one. The fasts method is real time kinematic. Final map scale is another one. The smallest object on the map (thickness of the line) is one tenth of millimeter, so e.g. for 1:1000 map scale, measurement precision up to 10 centimeters is acceptable. When we plan to work with grids, usually grid size is more than 1 meter, it means that measurement precision could be also up to used grid size.
Mapping
We are observing single points with GPS and after postprocessing of measurements we get their coordinates and ID's (name). But in GIS we recognize more types of spatial objects - points, lines and polygons. Attribute is other important feature connected to the spatial object. Is there any way, how to store all of this information directly during GPS measurement? One possibility is to buy professional software (or hardware) which allows us establish direct communication among GPS receivers and GIS software to produce maps (Arc News, 97/98). Other possibility is to create own system and to use already owned equipment.
The idea how to speed up and automates creation of the maps is to code ID of the measured points and to create program which will decode this ID's and generates necessary data and files. ID could consists from eight digits which can be used for coding. I have created following system of coding:
table no.3: code system
consecutive number of digits/stored information |
object name |
|||||||||
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
|||
code |
ID |
points |
||||||||
ID |
code |
number |
lines |
|||||||
ID |
+ |
code |
poly.label |
|||||||
ID |
* |
number |
poly.poly |
|||||||
ID |
= |
number |
poly.line |
ID = number of the object, number = number of the point created certain object, code = code for attribute
During GPS measuring in the field you should name points according mentioned scheme. After processing of the data you get file which contain coded name and coordinates.
Then you process this file with "decoding" program which create ASCII files need for generating of point, line, polygon and information layers by GIS program.
The last step is run macro program inside GIS which automatically build layers, create attribute tables, provide description or symbol to attribute code and generate the final map.
Conclusion
GPS is a modern and an effective tool for mapping purposes and can be used in broad number of applications. One of the advantage is possibility of automatic map creation, which can be used when other types of data sources are missing or does not contain all important information.
References