Which coordinate system to use?

By default, most GPS receivers and application software use the Lat-Lon-Altitude (LLA) geographic coordinate system based on the WGS84 datum. In this system, the horizontal positions are expressed in terms of angular measurements (degrees, minutes and seconds). As it is much easier to compute distances and areas using linear units in a Cartesian coordinate system, many GIS professionals and surveyors prefer to work with a plane projection, such as the State Plane system in the USA and the global UTM system.

iGPSGIS and iCMTGIS will let you  select the coordinate system and datum to use for displaying the position data. To do so, first tap the Main Menu icon. Select Setup then select Coordinate System. Pick the coordinate system of your choice.

If you will be using a built-in Map as a background map for data collection or navigation, then use the default LLA system.

To use the State Plane system, select “State Plane” and accept the displayed  “NAD83_CORS96/CSRS98”. Then select your state and the Region code, and enter the Zone number. Accept “US Survey Feet” as the distance unit.

The Lat-Lon-Altitude (LLA) system and the UTM system default to Meters. If you wish to work with distances in feet, then for the Unit field select “US Survey Feet” or “International Feet”. The difference between these two units is insignificant when working with non-survey grade GPS receivers.

Tap the green check mark at the top of the screen to save your settings.

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2 Responses to Which coordinate system to use?

  1. Michael S. says:

    Is there a way to search for a specific point by using feature data? For example find a water meter by searching the account number or meter number.

  2. cmtinc says:

    This functionality is in the CMT PC software (PC-GIS 09, PC-GPS 09, PC-MAPPER 09). It will be added to a future version of iOS app that we plan to release in 2012.

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