Gridded data associated with the various NAPGD2022 models can be downloaded here in the following formats: GGXF, *.bin, and *.b. Additional information about the regional grid extents, test cases, and reading/analyzing the data are also provided below.

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Test cases

A spreadsheet of test evaluations of the component models at selected points has been included so users can verify their correct implementation of these models or answer common questions about the datum. These test points include grid boundaries, major cities, and points of geographic and scientific interest.
Data Download

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Fortran code for reading the .b files

• real*8 xlatmin,xlonmin,dphi,dlam
• real*4 undu(5401,10801)
• read (*) xlatmin,xlonmin,dphi,dlam,ilat,ilon
• Do I=1,ilat
• xlat=xlatmin+(i-1)*dphi
• read(*) (undu(i,j),j=1,ilon)
• do j=1,ilon
• xlon=xlonmin+(j-1)*dlam
• ENDDO
• ENDDO

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Combining the static and dynamic fields

To compute the time-dependent geoid undulation, both the SGEOID and DGEOID grids must be interpolated to a user-specified latitude and longitude and then combined according to the integer modified Julian day (mjdn) of the computation time:

GEOID = SGEOID + DGEOID*(mjdn-58849)/365.25

The modified Julian day is determined from the year, month, and day of the date of interest according to the following algorithm:

• a = (14 - month)//12
• y = year + 4800 - a
• m = month + 12*a - 3
• jdn = day + (153*m+2)//5 + 365*y + y//4 - y//100 + y//400 - 32045
• mjdn = jdn - 2400001

The // symbol denotes integer division, e.g. 4//3 = 1 and 2//3 = 0. MJD 58849 corresponds to midnight, January 1st, 2020 or the 2020.0 epoch of the static geoid