Thread: Java Station and Channel objects provide different values for lat,lon,elev

Hi Doug,

As you say, usually the station and channel elevations agree. But their definitions are not the same and on occasion they differ for good reason. The station elevation is defined (in SEED) as "Elevation of local ground level in meters", whereas the channel elevation is defined as "Elevation of the instrument" followed by "To find the local ground depth, add the depth field to the instrument elevation". So the channel depth field should be the difference between these two elevations. The common case where these values differ are for borehole installations where the station elevation is the elevation of the ground and the channel elevations are down the hole where the actual sensors are located.

I think the case of CM.PRV and its channels is simply a metadata error. First off, the the channel elevation is higher than the station elevation, which makes no sense. Are the sensors on a tower!? Second, the depth value of 0 does not match the difference in elevations. So one of the two values is probably just wrong. As a data user I would pick the channel values over the station values.

Just to clarify, this is not specific to our web services or the Java IRIS-WS library, the values are directly from the metadata submitted by the network operator.

Thanks for reporting this, we'll contact the network operator to pursue a fix. Also, we will add a test to our upcoming metadata validator to flag such things and hopefully avoid the accumulation of more of these.

Chad

On May 6, 2016, at 10:27 AM, Doug Dodge <dodge1<at>llnl.gov> wrote:

Using Station web service Java API I notice that elevation, latitude, and longitude can all be obtained either from a Station object or from a Channel object. Usually these values agree, but in some cases they do not. When they differ should that be interpreted as the instrument being offset from the nominal station location?
An example is CM-PRV- *-*
MDA lists:http://ds.iris.edu/mda/CM/PRV/00?timewindow=2015/11/03%2000:00:00-2599/12/31%2023:59:59
Latitude 13.376000
Longitude -81.364000
Elevation 63

The Station object provides:
Latitude 13.376
Longitude -81.364
Elevation 63

The Channel object lists:
BHE-00 lat = 13.376200, lon = -81.363500, elev = 205.000000
BHN-00 lat = 13.376200, lon = -81.363500, elev = 205.000000
BHZ-00 lat = 13.376200, lon = -81.363500, elev = 205.000000
BHE-01 lat = 13.376200, lon = -81.363500, elev = 205.000000
BHN-01 lat = 13.376200, lon = -81.363500, elev = 205.000000
BHZ-01 lat = 13.376200, lon = -81.363500, elev = 205.000000

Notice that all channels report the same values and all disagree with the values from the station object. Is this an error?


Doug Dodge

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Could easily be an error, but SEED allows for channels that are not
exactly the same as the station location. Most common usage is
probably for a borehole sensor where the station is at the surface,
while the associated channel is down the hole. I think it has to be
within 1 km to be the same station, but there is nothing that enforces
this other than words in the spec.

In this case it could also be that the channel location is just more
accurate but they didn't want to update the station location as it
already existed. In general the channel location should give you the
best location, but many people just use the station location.

Philip


On Fri, May 6, 2016 at 1:27 PM, Doug Dodge <dodge1<at>llnl.gov> wrote:

Using Station web service Java API I notice that elevation, latitude, and
longitude can all be obtained either from a Station object or from a Channel
object. Usually these values agree, but in some cases they do not. When they
differ should that be interpreted as the instrument being offset from the
nominal station location?

An example is CM-PRV- *-*

MDA
lists:http://ds.iris.edu/mda/CM/PRV/00?timewindow=2015/11/03%2000:00:00-2599/12/31%2023:59:59

Latitude 13.376000

Longitude -81.364000

Elevation 63



The Station object provides:

Latitude 13.376

Longitude -81.364

Elevation 63



The Channel object lists:

BHE-00 lat = 13.376200, lon = -81.363500, elev = 205.000000

BHN-00 lat = 13.376200, lon = -81.363500, elev = 205.000000

BHZ-00 lat = 13.376200, lon = -81.363500, elev = 205.000000

BHE-01 lat = 13.376200, lon = -81.363500, elev = 205.000000

BHN-01 lat = 13.376200, lon = -81.363500, elev = 205.000000

BHZ-01 lat = 13.376200, lon = -81.363500, elev = 205.000000



Notice that all channels report the same values and all disagree with the
values from the station object. Is this an error?





Doug Dodge



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