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 Determining Your GPS Needs
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What is your GPS application?

When selecting a GPS product for any application, several factors need to be considered. Some of the most important factors are accuracy requirements, occupation time permitted and local environment at the site where the work will be conducted. For each GPS surveying or mapping application there are well-defined procedures for accomplishing either of these tasks. Listed below are some of the most common applications and procedures.

APPLICATIONS
 Control Survey   Topographic Survey   Stakeout   GIS Data Acquisition   Navigation 
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 PROCEDURE
 Static Surveying 
 Faststatic Surveying 
 Kinematic Surveying 
 RTK (Real Time Kinematic) Surveying 
 Differential GPS 
 Autonomous Operation 
  • Control Surveying is an application. Static and FastStatic surveying are two procedures that can be used to achieve it.
  • Topographic Surveying is an application. Kinematic surveying is a procedure that can be used to achieve it.
  • Mapping / GIS Data Acquisition is an application. Kinematic surveying is a procedure that can be used to achieve it.
  • Navigation application a receiver provides information about the device location, speed and heading. Utilizing other functions within the GPS controller software, the receiver allows the user to navigate to known locations with accuracy ranging from + 20 meters to + 1-2 cm. An essential component for navigation is the availability of a real time signal, preferably, RTCM, via satellite such as OmniStar or a ground base radio network such as the U.S. Coast Guard.
  • Other consideration when selecting a GPS product for an application is the method of differential correction or DGPS. Several methods can be applied to acquire DGPS for the final resulting coordinates. Post-processed GPS is one of the most commonly used approaches; however, in some applications real time DGPS will increase production and is necessary for accuracy navigation.

Cost of Accuracy

Definitions

Accuracy
Signifies the absolute nearness of measured quantities to their true values.

Occupation Time
Time you should remain at a location, logging GPS data. Occupation time can vary from 1 sec. to 1 hr. depending on application, environment and receiver design.

Environment
Surrounding environment such as trees, water, land or natural / manmade canyon.

Control Surveying
A control survey determines the coordinates of selected reference marks in certain geographic region. Control surveys normally produce very accurate coordinates, which use procedures that are time consuming and expensive.

Static Surveying
Static surveying requires simultaneous observations of at least 5 satellites. Data collection time is usually between 45 - 60 minutes, depending on atmospheric conditions and the number of satellites. Static surveying yields baseline components that are precise to better than + .5 mm + 1 ppm. Recommended base line limits are 40 kilometers or 24 miles or less.

FastStatic Surveying
FastStatic surveying requires simultaneous observations of at least 5 satellites. Data collection time is usually between 8 - 15 minutes depending on atmospheric conditions and the number of satellites. FastStatic surveying yields baseline components that are precise to better than + 1 cm + 1 ppm. Recommended base line limits are 20 kilometers or 12 miles or less.

Kinematic Surveying
Kinematic surveying requires simultaneous observations of at least 5 satellites. Data collection time is usually between 5 - 30 seconds, depending on atmospheric conditions and the number of satellites. Kinematic surveying yields baseline components that are precise to better than + 1 cm + 1 ppm. Recommended base line limits are 20 kilometers or 12 miles or less.

Topographic Surveying
Topographic surveys gather the coordinates xyz or xy of all essential points in a region of interest to produce a map representing the Earth or part of it.

Mapping/GIS Data Acquisition
Mapping / GIS Data acquisition is similar to topographic surveying, the emphasis is on efficiency rather than accuracy. Data collection time is usually between 1 - 30 seconds depending on atmospheric conditions and the number of satellites. The most significant difference between topographic surveying and mapping is that both coordinates and feature attribute information are collected at the same time. The data collected and stored on the data collector in what is known as a GIS compatible format. Features are represented using points, lines or area type entities.

DGPS
Determination of relative coordinates between two or more receivers, which are simultaneously tracking the same GPS signals. The purpose of DGPS is to reduce or remove errors due to the satellite clock, satellite position, atmosphere condition and SA when active. Differential GPS can be performed either in a post process manner or in real time using a radio communication link.

RTCM
The commission set up to define a differential data link to relay GPS correction messages from a reference or monitor station to a field user. RTCM SC-104 recommendations define the correction message format and 16 different correction message types.

 

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