Streamgage Datum Conversion

1. Streamgage Datum Conversion Eastern Region Height Modernization Consortium Webinar March 4, 2014 Jim Kolva, USGS, Office of Surface Water

2. Topics for Discussion • Problem from USGS Water perspective • Database Facts • How Height Modernization might help the conversion effort?

3. Problem • Most USGS streamgages have a datum surveyed or estimated to NGVD29. • Streamgages established since 2002 and surveyed using a Global Positioning System (GPS) should have datum set to NAVD88.

4. Problem

5. Problem

6. Problem

7. Problem • Mixing vertical Datums creates confusion; • This mixture of Altitude Datums has created a confusing situation for streamgage information users attempting to model flood plain inundation, establish flood profiles or use other hydrologic applications. • In some areas of the country, the difference between NGVD29 and NAVD88 can be over 4 feet.

8. Partner Applications of Information – NWS AHPS

9. Partner Applications of Information – NWS AHPS

10. USGS/NWS/FEMA Flood Inundation Initiatives

11. Policy Guidance from USGS NAVD88 is the recommended datum established in the Federal Geographic Data framework (Office of Water Information Technical memorandum 2002.01).

12. NAVD of 1988 >2,200 SW stations NGVD of 1929 >48,200 SW stations SW Station Datums in the USGS NWIS

13. Database facts • 84,944 Surface-Water sites • 62,032 determined elevation from Maps • 7,737 from Levels • 8,023 unknown source • 1,269 from a GPS • 4,771 already reported to NAVD88

14. Database facts • 84,944 Surface-Water sites • 62,032 determined elevation from Maps • Convert these to NAVD88 directly because within existing accuracy limitations unless a determination of need for more accuracy due to data use.

15. Database facts • 125 sites in NGS database (NGSIDB)for NAVD88 have a “gage” description. • Assumed to be USGS streamgages • Additional sites may be present but did not show up in the search • Approximately 17,000 sites need to have some analysis and conversion.

16. NGS assistance in GPS surveys • NGS is doing updating work and GPS surveys throughout the country • North Carolina and Michigan -- Will survey in at least one reference mark at active gages in work areas • Working on best ways to easily provide reference mark descriptions and gage locations to contractors doing this work and best way to report back to USGS.

17. Priority stations • USGS, NGS, USACE and NWS developing priority lists of gages for conversion • Pilot study in Missouri • MO has been working with Darrell Pratte, NGS Geodetic Coordinator with the Missouri DNR. Darrell has volunteered to serve in a role of oversight for field surveys and provide some personnel. • Plan was developed for conversion of gages in Missouri. Includes survey plans for those stations identified as needing that level of attention.

18. Funding Sources • Unfunded mandate to USGS Water Science Centers • Some will need GPS surveys to meet required accuracies or because of instability issues. • Can the Eastern Region Height Mod partner with the USGS efforts?

19. Issues to be resolved • Accuracy of the conversion. • Resources • Time and manpower • Money • Data already converted to elevation in database – Florida efforts

20. Questions and Comments??