An inventory of collocated and nearly-collocated
CGPS stations and tide gauges

Progress report on the survey
- (September, 2002) -




G. Wöppelmann, C. Boucher, M. Bevis, C. Bruyninx, P. Woodworth



Note : The dynamic tables below are continuously updated. Please, don't hesitate to complete the form given here or to sent an E-mail to Guy Wöppelmann (gwoppelm@univ-lr.fr) to help us to keep this survey up-to-date.


1. Introduction

In order to get a better idea about the status and opportunities of continuous GPS positioning of tide gauges, a survey is carried out within various geoscience communities since October 1999. The survey aims at identifying the existing permanent GPS stations which are close to tide gauges (up to approximately 10 km). Two progress reports have been released so far :

The 2001 progress report was presented to the GLOSS group of experts during its 7th meeting held in Hawaii, April 2001. An action item was agreed on there : to update the list of GPS receivers at tide gauge sites at regular intervals. Actually the list is continuously updated. However, an announce is sent through mailing lists before a new progress report is released.

Considering the involvement of DORIS teams in the monitoring of vertical land motions, with the aim of removing the land motion signals from the tide gauge sea level records, the present survey is extended to DORIS ground stations that fulfill the vicinity criteria. The survey might further be extended to other advanced geodetic techniques...


2. Survey results

The results of the survey are stored in a database whose contents can be browsed and searched through a web interface. Go to the following URLs to get a view of the survey tables :

The main views are ordered by station name. The 'Search' link presents a form that can be filled up with criteria. A subset of the data matching these criteria will then be displayed. The 'List all' link scrolls the view to the entire list of stations.

Few explanations on the table colums. Columns 2,3,4 are the name, longitude and latitude of the GPS (DORIS) station. The first column is an internal database index and the other ones are :

The codes represent an official commitment to a specific network. Therefore, they may give an idea of the quality, interest and data availability.


The following tables are simple text tables produced for the report in order to outline certain characteristics. (You just have to click on the links to access them).

General :

GLOSS related information :

Sorted by distance to the tide gauge :

Data availability, IGS and TIGA-PP :


The amount of operational permanent GPS stations less than about 10 km away from a tide gauge has increased from 129 to 153 since the last progress report. There are still 9 planned stations meeting the vicinity criterion. Overall, the stations are distributed worldwide (see figure 1), though not evenly. There is a bias towards the Northern hemisphere. The geographical bias is variable but is always worse when looking at restricted set of stations belonging to a particular application or technical theme (GLOSS, Less than 1 km,... see figures 2 and 3 for instance).

Figure 1

67 are tide gauge / GPS sites committed to GLOSS,

Figure 2

It is quite interesting to have a look at the distance between the GPS antenna and the tide gauge. An extended survey has shown that 57 GPS stations are less than 1000 meters away from the tide gauge (23 out of 57 are GLOSS ones, see map below), 6 stations are between 1 and 3 km and 25 are between 3 and 10 km. This feature is critical for some applications like GLOSS-LTT : In no circumstances can it be assumed that even relatively close sites are not moving differentially at the mm/year level.

Therefore, frequent levellings (at least annually) are required over a long period of time (10-20 years). Experience shows that these regular levelling surveys are often neglected over time, in particular if the distance involved is more than a few hundred metres. Where the distance is more than 1 km, it is unlikely anyone will perform a levelling tie on a regular basis. Moreover, the levelling error can become a significant part of the total error budget. So, stations more than 1 km away cannot be considered 'nearly collocated' in the practical sense, though they may still be of interest for other applications (GLOSS-OC...).

It is important to say that the survey is far from complete since the distance information is missing for 65 stations...

Figure 3

Figure 4 shows the stations for which GPS observations can be retrieved freely at an identified data center on Internet. 67% of these stations contribute to IGS and are labeled as IGS Tracking stations. The percentage increases to 85% if EUREF stations are taken into account.

Figure 4


3. Outlooks

The survey highlights that data of almost one third of the stations seem not available yet (46 out of 153). Of course, all these stations may not be useful for scientific applications. A close inspection should therefore be undertaken. More information is also needed to actually take benefit of CGPS@TG site collocations, for instance :


4. Related useful documents on the web