- 1. Will the current NAD 83(2007) designation remain or will it change to NAD 83(NSRS2007)?
- The official "name" or "designation" of the adjustment that was undertaken in 2007 is NAD 83(NSRS2007). The shortened "tag" of NAD 83(2007) was adopted for ease of formatting the NGS data sheets and will remain on the data sheets.
- 2. Is the 2007 National Readjustment compatible with the Canadian Spatial Reference System?
- Yes, the Canadian Spatial Reference System (CSRS) is compatible with the 2007 National Readjustment horizontal positions and ellipsoidal heights. This compatibility was achieved by incorporating Canadian Active Control Point sites which were transformed into NAD 83 through the same 14 parameter transformations agreed upon by the Natural Resources Canada´s Earth Sciences Sector and NGS. Compatibility also depends on how velocities are determined between the two systems and how they are applied in areas affected by plate tectonics, subsidence or glacial rebound. Users must be careful in applying accurately determined coordinates for a specific epoch in time.
Note: The CSRS utilizes a total gravimetric geoid instead of GEOID03 for their orthometric height system. Therefore an inconsistency may result when applying different geoid models to the readjusted ellipsoid heights in determining GPS-derived orthometric heights.
- 3. I see the new datum tag for our HARN stations is NAD 83(2007). Will the data sheets for the CORS stations also get this datum tag?
- The CORS data sheets will not change. The datum tag will remain NAD 83 (CORS) with an epoch date of 2002.00 for all CORS other than those
located in Alaska where the epoch date is 2003.00 to post-date the November 2002 Denali earthquake.
- 4. What is the relationship between the new NAVD88 orthometric heights from the 2007 National Readjustment and the heights derived with GEOID03 in the field?
- The NAD 83(NSRS2007) National Readjustment only computed the ellipsoidal heights on stations which had GPS observations. GPS-derived orthometric heights shown on the NGS datasheets are determined in one of two ways. The first and primary method of determining GPS-derived orthometric heights are through a project which rigorously constrained published local bench marks. The second, and rarely used method, applies the current geoid model to the adjusted ellipsoid heights. Since GEOID03 was developed by fitting the gravity geoid using the previous determined NAD 83(HARN) set of ellipsoid heights, applying GEOID03 to the current set of ellipsoid heights from the NSRS2007 will be different. The differences may be small over much of the country but not everywhere. The next geoid model currently being developed by NGS, expected to be available by the end of 2008, will incorporate the latest ellipsoid heights from the NSRS2007 adjustment.
Note: Orthometric heights in Alaska are determined by applying the current GEOID06 model to the new NSRS2007 ellipsoid heights.
- 5. Are NAD 83(CORS96) coordinates compatible with NAD 83(NSRS2007) coordinates?
- NGS has adopted a realization of NAD 83 called NAD 83(NSRS2007) for the distribution of coordinates at ~70,000 passive geodetic control monuments. This realization approximates (but is not, and can never be, equivalent to) the more rigorously defined NAD 83(CORS96) realization in which Continuously Operating Reference Station (CORS) coordinates are distributed. NAD 83(NSRS2007) was created by adjusting GPS data collected during various campaign-style geodetic surveys performed between the mid-1980´s and 2005. For this adjustment NAD 83(CORS96) positional coordinates for ~700 CORS were held fixed (predominantly at the 2002.0 epoch for the stable North American plate, but also at the 2007.0 epoch in Alaska and western CONUS) to obtain consistent positional coordinates for the ~70,000 passive marks. Derived NAD 83(NSRS2007) positional coordinates should be consistent with corresponding NAD 83(CORS96) positional coordinates to within the accuracy of the GPS data used in the adjustment and the accuracy of the corrections applied to these data for systematic errors, such as refraction. In particular, there were no corrections made to the observations for vertical crustal motion when converting from the epoch of the GPS survey into the epoch of the adjustment while the NAD 83 (CORS96) coordinates do reflect motion in all three directions at CORS sites. For this reason alone, there can never be total equivalency between NAD 83(NSRS2007) and NAD 83(CORS96).
Note: NGS has not computed NAD 83(NSRS2007) velocities for any of the ~70,000 passive marks involved in this adjustment. Also, the positional coordinates of a passive mark will make reference to an “epoch date.” Epoch dates are the date for which the positional coordinates were adjusted and are therefore considered “valid” (within the tolerance of not applying vertical crustal motion). Because the positional coordinates of a mark will change due to the dynamic nature of the earth´s crust, the coordinate of a mark on epochs different than the listed “epoch date” can only be accurately known if a 3-dimensional velocity has been computed and applied to that mark.
- 6. Now that the NAD 83(NSRS2007) has been loaded and published, will NGS continue to accept and load projects which were constrained to stations determined by earlier published values (i.e., state High Accuracy Reference Network or Federal Base Network projects)?
- Beginning June 15, 2008, NGS will only accept projects which have been adjusted to the current realization of the NAD 83 in the survey area.
Note: Current realizations are:
- NAD 83 (NSRS2007) or NAD 83 (CORS96) for US, Alaska, Puerto Rico and the Virgin Islands
- NAD 83 (93) for Hawaii and Guam
- NAD 83 (02) for Pacific-American Samoa and the Northern Marianas
- 7. Will NGS readjust projects constrained to earlier epochs and load them into the NGS database to the newly published NAD 83(NSRS2007) coordinate values?
- NGS requests that projects submitted after November 2005 be readjusted to NAD 83(NSRS2007) by the user and resubmitted to NGS. NGS currently does not have the resources to undertake this task.
- 8. I have coordinates on different realizations. How do I transform between NAD27, NAD 83(1986), NAD 83(HARN) and NAD 83(NSRS2007)?
- NADCON will transform any coordinates on NAD27 or NAD 83(1986) to NAD 83(HARN). The shifts from NAD 83(HARN) to NAD 83(NSRS2007) are in the 1 to 3 centimeter range. Initial tests indicated that the shifts were too small, the accuracy of the shifts too large, and sparseness of the data too great to produce a useful transformation to NAD 83(NSRS2007). Users who wish to transform existing surveys from an older version of NAD 83 into NAD 83(NSRS2007) are urged to re-compute coordinates using original observations and newly published NAD 83(NSRS2007) coordinates.
- 9. My contract requires me to tie to first-order or A-order stations. When I look at the new data sheet there is no longer a Horizontal order. How do I know if a control point meets first-order or A-order criteria?
- The new datasheets will continue to show the order/class codes in the superseded section.
- 10. How will we produce and incorporate network and local accuracies in new project adjustments?
- NGS is investigating the proper approach for incorporating and producing accuracies in future adjustments.
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