# GPS Height

The GPS Height tab is used to specify heights and height units to be used by ArcPad when calculating z-coordinate values.

ArcPad calculates heights in one of two formats:

- Height above ellipsoid (HAE). This is the default.
- Height above Mean Sea Level (MSL) if a geoid separation value is specified.

To specify appropriate values for the GPS height fields it is necessary to understand how ArcPad calculates heights, or zvalues.

GPS receivers output the height of the GPS antenna, in meters, via the $GPGGA sentence of the NMEA protocol. This height is relative to Mean Sea Level (MSL). Many GPS receivers also output the geoidal separation, in meters, via the same $GPGGA sentence. The geoidal separation is the distance between the geoid and ellipsoid. ArcPad then calculates the sum of the GPS antenna height (relative to MSL) and the geoidal separation to produce the ellipsoidal height (HAE, Height Above Ellipsoid) which is potentially stored as the z-value. So from the $GPGGA message, ArcPad calculates HAE using the following formula:

HAE = MSL_Height + GeoidSeparation

This HAE is based on the datum used by the GPS receiver, which is typically WGS84. The default units for HAE are meters. When the TSIP protocol is used, ArcPad requests the HAE height from the GPS; consequently, no calculation of the HAE is needed by ArcPad.

To store accurate z-values, it is necessary to provide ArcPad with additional information to be used for calculating the Z-values. Antenna Height: Since the height reported by the GPS is actually the height of the GPS antenna, it is necessary to let ArcPad know the height of the GPS antenna above the ground. ArcPad then subtracts the antenna height from the GPS height in to determine the z-value for the GPS position at ground level.

## Geoid Separation

The height calculated by GPS receivers is HAE, in meters. To calculate the MSL height, GPS receivers store a lookup table of geoid separation values. Unfortunately, due to storage constraints on the GPS receiver, these geoid separations are not the most accurate. Consequently, the MSL height output in the $GPGGA sentence could be based on geoid separation values that are not sufficiently accurate for your height accuracy requirements. The geoidal separation is not a constant but varies across the earth and with time. Accurate geoidal separations are available from national geodetic agencies. For example, in the conterminous United States a GEOID03 geoid height separation can be computed for latitude-longitude coordinates at the following Web site:

The geoid separation computed from the above Web site is more accurate than the geoid separation stored in GPS receivers.

Specifying a Geoid Separation in ArcPad enables ArcPad to calculate a more accurate MSL height than the MSL height reported in the $GPGGA sentence. The formula ArcPad uses to calculate z-values is:

z-Value = HAE - PrefsGeoidSeparation - PrefsAntennaHeight

where PrefsGeoidSeparation is the Geoid Separation specified on the GPS Height tab of the GPS Preferences dialog box in ArcPad, and PrefsAntennaHeight is the Antenna Height specified on the GPS Height tab of the GPS Preferences dialog box in ArcPad. It should be noted that you should only specify a Geoid Separation in ArcPad if all the following are true:

- You have an accurate geoid separation value for the area where you will be working with ArcPad.
- You need to store z-values as MSL height.
- You are using a professional-grade GPS receiver.

To summarize:

- By default, ArcPad stores z-values as ellipsoidal height (HAE).
- When a geoid separation is provided, ArcPad stores z-values relative to Mean Sea Level height (MSL).

## Antenna Height

Enter the height of the GPS antenna, in the specified height units. ArcPad subtracts the antenna height from the ellipsoidal height (HAE) to determine the height for the z-value.

The default is 0.

## Geoid Separation

Enter the distance between the geoid and ellipsoid, in the specified height units. ArcPad subtracts the geoid separation from the ellipsoidal height (HAE) to determine the height for the z-value. The resultant height is the MSL, or orthometric height. The Geoid Separation is negative where the geoid lies below the ellipsoid.

The default is 0.

## Use Map Units for Height Units

When checked, the map units are used for the height units. If the map units are degrees, then the height units will be meters. The Height Units drop-down list is unavailable when Use Map Units for Height Units is checked.

When unchecked, the height units used by ArcPad will be the units chosen in the Height Units drop-down list.

The height units specified with this check box and the Height Units drop-down list are used in all ArcPad calculations and display of elevation and z-values, including the Vertical Distance of the Point/Vertex Offsets dialog box, and the Elevation field of the GPS Position Window.

The default is checked, and consequently, the Height Units dropdown list is unavailable.

## Height Units

Choose the height units to be used by ArcPad when calculating z-values.

The following units are available: Centimeter: cm, Chain: ch, Chain_Benoit: ch Benoit, Chain_Sears: ch Sears, Chain_US: ch US, Decimeter: dm, Fathom: fm, Foot: ft, Foot_Clarke: ft Clarke, Foot_Indian: Ind ft, Foot_MA: ft MA, Foot_Sears: ft Sears, Foot_US: ft US, Inch: in, Inch_US: in US, Kilometer: km, Link: lk, Link_Benoit: lk Benoit, Link_Clarke: lk Clarke, Link_Sears: lk Sears, Link_US: lk US, Meter: m, Meter_German: GLM, Mile_Statute: mi, Mile_US: mi US, Millimeter: mm, Nautical_Mile: nm, Nautical_Mile_UK: nm UK, Nautical_Mile_US: nm US, Rod: rd, Rod_US: rd US, Yard: yd, Yard_Indian: Ind yd, Yard_Sears: yd Sears, Yard_US: yd US.

The default is deactivated. When Use Map Units for Height Units is unchecked, the default is Meter: m.

## Use Height in Datum Transform

When checked, the height calculated by the GPS receiver is used as part of the datum-to-datum transformation. When unchecked, 0 is used.

The default is unchecked.