egm96geoid

Read 15-minute gridded geoid heights from EGM96

Syntax

[Z, refvec] = egm96geoid(samplefactor)
[Z, refvec] = egm96geoid(samplefactor,latlim,lonlim)

Description

[Z, refvec] = egm96geoid(samplefactor) imports global geoid height in meters from the EGM96 geoid model. The data set is gridded at 15-minute intervals, but may be down-sampled as specified by the positive integer samplefactor. The result is returned in the regular data grid Z along with referencing vector refvec. At full resolution (a samplefactor of 1), Z will be 721-by-1441.

The gridded EGM96 data set must be on your path in a file named 'WW15MGH.GRD'.

[Z, refvec] = egm96geoid(samplefactor,latlim,lonlim) imports data for the part of the world within the specified latitude and longitude limits. The limits must be two-element vectors in units of degrees. Longitude limits can be defined in the range [-180 180] or [0 360]. For example, lonlim = [170 190] returns data centered on the dateline, while lonlim = [-10 10] returns data centered on the prime meridian.

Background

Although the Earth is round, it is not exactly a sphere. The shape of the Earth is usually defined by the geoid, which is defined as a gravitational equipotential surface, but can be conceptualized as the shape the ocean surface would take in the absence of waves, weather, and land. For cartographic purposes it is generally sufficient to treat the Earth as a sphere or ellipsoid of revolution. For other applications, a more detailed model of the geoid such as EGM 96 may be required. EGM 96 is a spherical harmonic model of the geoid complete to degree and order 360. This function reads from a file of gridded geoid heights derived from the EGM 96 harmonic coefficients.

Examples

Read the EGM 96 geoid grid for the world, taking every 10th point.

[N,refvec] = egm96geoid(10);

Read a subset of the geoid grid at full resolution and interpolate to find the geoid height at a point between grid points.

[N,refvec] = egm96geoid(1,[-10 -12],[129 132]);
n = ltln2val(N,refvec,-11.1,130.22,'bicubic')

n =
	52.7151

More About

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Tips

This function reads the 15-minute EGM96 grid file WW15MGH.GRD. The grid is available as either a DOS self-extracting compressed file or a UNIX® compressed file. Do not modify the file once it has been extracted.

    Note   For details on locating map data for download over the Internet, see the following documentation at the MathWorks Web site: Finding Geospatial Data.

Maps will extend a half a cell outside the requested map limits.

There are 721 rows and 1441 columns of values in the grid at full resolution. The low resolution data in GEOID.MAT is derived from the EGM 96 grid.

See Also

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