OMG.?ÿ
I just received this notice:
Urgent: Critical Change to GEOID18 Grid Files
NGS has corrected errors in some of the GEOID18 grid files posted on the GEOID18 Downloads webpage prior to November 26, 2019.?ÿ See below for more information and details.
Critical Change
The GEOID18 grid files in the little endian and ASCII grid formats that were posted on the NGS website prior to November 26, 2019, contained errors in a small percentage of the grid cells (1 in 500). NGS has since corrected the errors and replaced these files on the GEOID18 download page. We encourage users that have previously downloaded those grids to immediately replace them with the updated grids.?ÿ
Unaffected Products
The GEOID18 model itself, the big endian file format, and all of the NGS tools and services that incorporate GEOID18, including the Online Positioning User Service (OPUS), remain unaffected and may continue to be used.
Technical Details
The GEOID18 grid files posted prior to Nov 26, 2019 were created and tested in the big endian format. The grids were then converted into little endian and ASCII formats. Unfortunately, an error in the file format conversion process introduced errors into a random set of grid cells within the little endian and ASCII grids. These errors range from 1.9 mm to 6.25 cm and are inconsistently distributed throughout the grids.
To help avoid these types of issues in the future, NGS is working to adopt a single grid file format sometime in 2020.?ÿ
Background
NGS creates geoid models to help users determine GPS-derived orthometric heights, and the most recent published model is GEOID18. NGS incorporates geoid models into other tools, and makes the models available for download in three grid file formats: big endian, little endian and ASCII. Surveying equipment and software vendors also download and incorporate NGS geoid grid files into their products.
The problem that I have is I don't know which data sets derivative GEOIDS (like those provided by Trimble, Carlson and others) were based on.
This is like "D??j?ÿ vu all over again".?ÿI spent days agonizing over 18 cm differences east of St. George Utah and out in the West Desert. I guess I will need to revisit.
I think that to keep all this straight I am going to have to rebuild all the G18 files that I use and mark them with G18B.
I had not used GEOID18 in production yet, but was getting ready to send out a project today that used it in the northern plains. I checked, and yes there were scattered points that were slightly different, but only up to 0.004 m. So no big deal. But they said it can range up to 6+cm. One problem as I see it right now is that they use the same file name for both big endian and little endian, so I was not exactly sure which one I had used until I downloaded the new little endian (dos system).?ÿ
I just loaded Geoid18 from Trimble......guess I need to follow up on that. Thanks for the heads-up.
URGENT for users of SurvCE Version 6!
The NGS published a bad GEOID18 implementation on their website.
This file was used to generate the SurvCE Version 6 GEOID18.
If you use SurvCE with G18, please read this alert [?ÿGeoid18 Verification in Carlson SurvCE?ÿ] ASAP.
Your best course of action is to revert to GEOID12B at this point.
Makes me wonder...was this ever a problem in past geoid models??ÿ
