Copyright © 2010 Leica Geosystems Inc. – All rights reserved

Leica Geo Office CombinedOctober 2010

Creating a Geoid Field File Summary A Geoid is the equipotential surface of he Earth’s gravity field which best fits, in a least squares

sense, global mean sea level. The Geoid Field File enables the user to utilize a Geoid Model that is appropriate for the mapping are under consideration. With a Geoid Model attached to a Coordinate System the user can compute Geoid Separations of the points in a project. If Geoid Separations are available it enables the user to switch between viewing Ellipsoidal and Orthometric heights. The relationship between these two heights is given by: Ellipsoidal Height (h) = Orthometric Height (H) + Geoid Separation (N) This quick guide will go through the procedure of properly creating a Geoid Field File (.GEM) in LGO. The first method will be from a Center and Radius, and the second method will be through the extents of an area to be covered by the field file. Once the geoid field file (.gem) is created this quick guide will illustrate the uploading of the Geoid Field File onto the CS10/15 through two methods: 1) data card inserted directly into the computer 2) ‘My Device’ with the field controller directly connected to the computer via active sync

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Leica Geo Office CombinedCreating a Geoid Field File

Notes Geoid Models are always an approximation of the actual Geoid. In terms of accuracy, they may vary considerably and in particular global models should be used with care. If the accuracy of the Geoid Model is not known it might be safer to use local control points with Orthometric heights and apply a transformation to approximate the local Geoid.

Creating Geoid Field File

It is assumed that the user has already created a Geoid Model in LGO. Without a Geoid Model, the user will be unable to create a Geoid Field File.

Procedure: Center and Radius

From the Tools drop down box in LGO, select Create Geoid Field File

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Leica Geo Office Combined Creating a Geoid Field FileProcedure: Center and Radius

Using a mapping tool like Google Earth is a good visual aid to estimate the central location and the radius of which the Geoid Field File is to cover.

The Geoid Field File uses a portion of a Geoid model to create the file. Select the Geoid Model, the ‘Centre & radius’ radio button and pick the interpolation method to be used. Use the determined geodetic coordinates for the center as well as the radius of the covered area. Make sure you are using a West longitude for North America. Select the spacing for this area and select save to generate the .GEM file.

Points to consider

In choosing a Geoid Model, be aware of the reference ellipsoid. When attaching the .gem file to a coordinate system, the reference ellipsoid must be the same. In choosing an interpolation method, remember that Geoid 99 and 03 both use a bi-quadratic interpolation whilst Geoid 09 uses a spline interpolation. Regarding spacing and radius. The minimum spacing of interpolation is 1000 m. The normal spacing for larger areas is 2000 m. Use a minimalist in spacing, keeping a watch on the size of the file that will be created.

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Leica Geo Office Combined Creating a Geoid Field File

Saving created file

You will next need to save the .GEM file to a location on your local or network drive.

Procedure: Extents

The Extents option would be more suitable for creating Geoid Field Files of larger areas.

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Leica Geo Office Combined Creating a Geoid Field File

Procedure: Extents

With using the Extents option, the southwest corner and the northeast corner of the coverage rectangle is needed. Please make sure you are using a West longitude for North America. As stated earlier, for larger areas, the spacing should reflect as such.

Saving Geoid Field File

As mentioned previously, the file can be saved to any location on the local computer or network drive.

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Leica Geo Office Combined Creating a Geoid Field File

Procedure: Loading Geoid Field File onto Viva CS 10/15

In the directory of the Geoid Field File (.gem), copy the created file.

Methods of loading Geoid Field Files

Method 1: With the SD card or CF card that is to be used in the CS controller inserted directly into the computer, copy the .GEM file to the \Data\GPS\Geoid on the memory card.

Method 2: With an ActiveSync or Windows Mobile Device Center connection, explore to the field controller and copy the .Gem file onto \Data\GPS\Geoid folder on the SD card or CF card being used in the CS controller.

Transfer Gem file onto CS Controller

With the gem file in the correct folder and at the main menu of Smartworx, press 4 (User) > 4 (Tools & utilities..)

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CS 10/15 and GS 10/15 Creating a Geoid Field FileTransfer Gem file onto CS Controller

Press 1 (Transfer user objects) In the transfer objects screen select the Geoid field file from the pull down, specify what medium the Geoid field file is stored and transfer the file to the internal memory.

With the required fields populated, the name of the Geoid field file will be available in the Geoid Field file pull-down. Select the correct file and press F1 (OK). Once copied, the instrument will displayed that it the transfer was successful.