# Presentation: 663529

## Status and Plans of the National Geodetic Survey’s Gravity Database Update

- Daniel R. Roman and Yan Ming Wang
- October 13-14, 2005 Austin, Texas

**Notes:**
This was a joint NOAA-NRL-NASA effort designed to satisfy a number of institutional needs.
 I’d like to thank my co-authors and their support staffs for all their hard work on this.
 What I’m covering today represents the results from the first of three studies to be covered in the next few years. This is still a work in progress and will serve as an update on our progress.
 It has been funded by the National Ocean Service’s Partnership Proposal program in an effort to refine the gravimetric geoid model in littoral regions and to better understand the nature of the errors associated with the North American Vertical Datum of 1988.
 NAVD 88 is the official U.S. vertical datum from which all heights on topographic charts are measured. 
 Meter-level biases and trends are thought to exist within NAVD 88 – bringing into question its utility for emergency and coastal management.

## DISCUSSION OUTLINE

- NGS-held point surface gravity measurements
- Problems in the current gravity database
- The first effort to fill in data gaps near shore
- Future Plans
- Invitations

**Notes:**
 In the Overview, I’ll cover what we hope to gain from this study and possible impacts.
 Next, I’ll cover the mission collection – specifically regarding the gravity data.
 With a better established gravity field, analysis of the NAVD 88 datum is possible.
 Next, I’ll cover where we intend to go in the near term and beyond.
 Last, I’ll summarize this oral presentation.

## NGS-held point surface gravity measurements

- 2,155,662 surface gravity measurements scattered throughout North America
- mixture of NGS & NGA-proprietary data

## Slide 4

## Problems in the current gravity database: 

- Tidal systems differ between NGS (tide-free) and NIMA-proprietary (mean tide) data
- Data reference NAD27 & NGVD29 but can be converted to NAD83 & NAVD88
- Data collected for a long time period (dating back more than 80 years), may contain local or regional biases
- Effect of subsidence and the Earth’s crustal movement on gravity has become considerable
- Data gaps in near shore and difficulty-to-access areas

## The first effort to fill in data gaps near shore:

- NGS/NRL/GSFC joint airborne gravity and LIDAR mission in Golf of Mexico region
- Data were collected in May/June 2005
- The misson’s goals are:
- 	- To fill in data gaps near shore
- 	- Establish links between off-shore mean sea surface and the North America Vertical Datum 88 (NAVD88)

## GRAVITY-LIDAR COLLECTION MISSION

**GRAVITY-LIDAR COLLECTION MISSION**

**Notes:**
A mission (GLS05) was planned to estimate a consistent grid with sufficient crossovers to determine the regional gravity field. 
PLANNED PROFILES: GRAVITY AND LIDAR
Off of Southwestern Coast of Florida & North of Cuba
Profiles from 50 km inshore to 200 km offshore
Track spacing of 10 km
Cross tracks at 50-100 km spacing
Profiles were flown at about 28,000 ft (8.5 km)
Florida data will bridge gap in near shore regions
Cuba data will provide constraints at edge of solution

What we hoped to get ...

## Slide 8

**Notes:**
... and what we actually got. Note that this is only the gravity field recovered along the profiles. LIDAR coverage is even poorer due to extensive cloud cover in the region. Bad weather during Hurricane season and other factors negatively affected the collection process and limited the scope of the resulting models to about 40 km full wavelength.

## Slide 9

**Notes:**
Gravity data at 28,000 feet collected during GLS05. Block-meaned at 1/8 degree, then minimum curvature spline.

## Slide 10

**Notes:**
Difference between upward continued surface gravity (using GRIDFFT from GMT) and aerogravity grid (min. curvature). In general, there is a -18 mgal bias present but, otherwise good agreement. Many of the remaining features are likely artifacts from the rough data and require further processing.

## Future Plans:

- The next study area will flow from the Florida panhandle through Louisiana into Texas along the Gulf coast
- Airborne missions over coastal and complete US continental are proposed
- A complete coastal and inland airborne survey is proposed
- Missions will be at 20 km cross track spacing and at 8.5 km altitude

## Invitations:

- We are seeking corporations between government agencies, such as NGA, NRL, NASS and USGS.
- Research institutes and universities are welcome.
- Private industry (oil companies and mapping companies) are invited to participant.