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Conversion of Tampa Stations to Common Datum of NAVD 1988

Project Chief: David L. Fulcher
Cooperator: Southwest Florida Water Management District (SWFWMD)
Period of Project:October 2010 – September 2012

Problem Statement

Scientists use geospatial data obtained from many sources, and datums and projections often must be standardized before datasets that are in different or unknown datums and projections can be used. The widespread availability of multiple datums means it is important to record and publish datum information for any document that contains vertical locations. The National Geodetic Vertical Datum of 1929 (NGVD 29) is a fixed datum that was adopted as a national standard geodetic reference for elevation. It is now considered superseded as the national standard by the North American Vertical Datum of 1988 (NAVD 88), which provides a more accurate representation of the Earth's shape and a more accurate depiction of the location of objects in North America than previous datums. Use of NAVD 88 for geospatial products is increasingly common.

The U.S. Geological Survey (USGS) National Water Inventory System (NWIS) is the primary data base for acquisition, processing, and long-term storage of hydrologic data collected by USGS. NWIS will only allow the display of the current datum in use for a station. In most cases this datum has been NGVD 29, but for some sites the datum was set at an arbitrary datum. Consequently when the public and other users download data from NWIS, the water-level data may be set to multiple datums. The user must then apply corrections to the data to adjust to a common datum. The Southwest Florida Water Management District (SFWWMD) is a frequent and continuous user of NWIS data, and would prefer that all data accessed in NWIS be available at NGVD 88. However the NWIS data base can only be upgraded by USGS at a national level and that effort, while underway, is not yet complete. Therefore, the Florida Water Science Center was asked by SWFWMD to develop a computer program (script) that will allow conversion of NWIS water level data to NAVD 88 and display the converted data on a web page.

Objectives

1) Upgrade daily value water-level data in NWIS that use multiple datums to the common datum NAVD 88.
2) Display water-level data from NWIS on a web site, such as NWISWEB or the Hydrologic Web Portal, using a common datum.
3) Provide the basis for the future upgrade of the USGS NWIS system to automatically handle multiple datum offsets in future software revisions.

Approach

Gaging stations and wells are tied into benchmarks having an established elevation (in reference to NAVD 88) by differential leveling. The differences between the station’s reference marks at the current gage datum and NAVD 88 were used to determine the conversion value required to convert the station to NAVD 88. Historical daily values of gage height and elevation were converted to NAVD 88 by means of a script which pulls the historical values from the database, applies the conversion value, and then reenters the NAVD 88 value under a different data descriptor. In addition, any currently active real-time stations are updated to display gage height and elevation at both gage datum and in reference to NAVD 88 on NWISWEB. Following conversion, all station manuscripts are updated to reflect the station’s gage height or elevation in reference to NAVD 88 for the purposes of publication in the Annual Data Report.

Results

To date approximately 95% of all groundwater stations active at the beginning of the project have been converted to NAVD 88. Approximately 33% of all surface water stations active at the beginning of the project have been converted to NAVD 88, and it is anticipated that the remaining 67% will be converted by the end of the 2012 water year.

Information Products

The project will result in a separate retrievable data set of either stream elevation or groundwater elevation above NAVD 1988 for each station via NWISWeb. Historical daily values converted to NAVD 88 will be retrievable from the start of the period of record. Unit values converted to NAVD 88 will be available from the date that the conversion was carried out.