Epoc Date?

[foxtrot_xray]

On a lot of datasheets, there's an Epoc Date:

 ED3723  EPOCH DATE  -		2002.00

.. Obviously, the 2002 is the year. What's the .00? The week of the year? I THOUGHT this, until I saw one with .65 after it. So, then, is the ".00" a percentage? (i.e. ".50" would mean 182 days into the year?)

 

I always remember seeing it, but never thought about it.

[+Klemmer]

Maybe they are like Stardates?

[Bill93]

Yes, decimal years. It has long been common practice in astronomy for use in adjusting the star catalogs from a publication date to the current date.

Edited March 2, 2009 by Bill93

[foxtrot_xray]

Eenteresting. Thanks guys.

 

Just curious, if that's the case, how come the history logs aren't done like a stardate? ("Captain's log..")

[+Klemmer]

Sheesh! I was just kidding! They really are like stardates? Can't be that easy, can it?

[68-eldo]

From Wikipedia

 

In the fields of chronology and periodization, an epoch means an instant in time chosen as the origin of a particular era.

 

I think in computer terms it can mean from the time of installation of the program. But that does not seem to be the case here. You would have to ask NGS what it means.

[+billwallace]

From Wikipedia

In the fields of chronology and periodization, an epoch means an instant in time chosen as the origin of a particular era.

I think in computer terms it can mean from the time of installation of the program. But that does not seem to be the case here. You would have to ask NGS what it means.

Ahhh ... so that's what ephemeris means.

[+GEO*Trailblazer 1]

I saw something once or twice about the GPS Epoch date.

I think it was a start to the GPS timing in ????.

 

I will have to relook that up.

 

Epoch - A specific instant in time. GPS carrier phase measurements are made at a given frequency (e.g. every 30 seconds) or epoch rate.

 

Epoch Date - The date, usually expressed in decimal years, for which published coordinates and

data are valid.

 

Edit few of em......

 

2.

The current standard epoch is called "J2000.0" (Julian 2000, based on the Julian Calendar), is defined by international agreement, and is precisely defined to be approximately noon at the Royal Observatory, Greenwich, in London England:

 

The Julian date 2451545.0 TT (Terrestrial Time), or January 1, 2000, noon TT.

This is equivalent to January 1, 2000, 11:59:27.816 TAI (International Atomic Time) or

January 1, 2000, 11:58:55.816 UTC (Coordinated Universal Time).

 

3.

Epoch (geology)

In geology, an Epoch is a time division criteria based on specific physical and chemical characteristics in rock layers (Lithography) indicative of the global environment, including tectonic activity (mountain building, continental drift, etc.) and include macroscopic (visible by inspection) and microscopic Fauna features or characteristics that are clear and distinct from other series defining criteria. For example, the formations, rock beds, and members of differing rock types that were being laid down in different environments at the same time.

Each defined set of characteristics—in a word, Epochs —directly correspond to specific rock series, the equivalent of an "era" (both are measured usually in time units) in rock layers. Since such layers correspond to time (or era) of formation, they are used to date pre-historic events in other sciences.

Edited March 3, 2009 by GEO*Trailblazer 1

[Mesa Mike]

The GPS time is measured in seconds since the GPS epoch -- 0000 UTC, January 6, 1980.

See this article.

 

Why benchmarks would need anything at all to do with GPS epochs is beyond me.

Edited March 3, 2009 by Mesa Mike

[+GEO*Trailblazer 1]

I know from talking to DaveD the Tidal marks have epochs.

 

The average mean is calculated over the epoch.

 

Now to go deeper you will need to talk to him.

[Papa-Bear-NYC]

Why benchmarks would need anything at all to do with GPS epochs is beyond me.

Since sometime in the 1980s most, if not all survey markers (of the Geodetic, not cadestral variety) have been measured with high quality GPS. Furthermore the latest NGS adjustment (2007) was based on GPS stations. So modern survey marker measurements and datums have everything to do with GPS epochs.

[Bill93]

GPS operates in WGS84. WGS84 is tied to the average of positions on all the world's tectonic plates. NAD83 is tied to the average of positions on the major North American tectonic plate. The NA plate moves with a velocity of 1 or 2 cm per year relative to the average of all of the earth's surface. So any conversions between GPS readings and NAD83 at that level of precision need to define the epoch of the measurements being used for the conversion.

 

That's what I think I know. Wish I understood it better. Here's a link to a discussion on a professional forum where they mention adjusting to epochs. Even the people who know how to use the tools don't always know what's behind them.

Edited March 3, 2009 by Bill93

[foxtrot_xray]

The NA plate moves with a velocity of 1 or 2 cm per year relative to the average of all of the earth's surface. So any conversions between GPS readings and NAD83 at that level of precision need to define the epoch of the measurements being used for the conversion.

Most of ya'll's chatter went far over my head, but THIS I understand - or think I do, at least, due to this quote from the datasheet file:

Epoch dates are the date for which the positional coordinates were adjusted, and are therefore considered "valid" (within the tolerance of not applying vertical crustal motion).

[AZcachemeister]

I was sure it had something to do with continental drift...

I'll check my Tri-corder and get back to you.

[DaveD]

In the context of geodesy the term epoch can have two meanings. One is the length of time of a particular set of observations. For example, GPS measurements can be said to have an epoch rate of collection of 1, 5, 15, 30 seconds. etc. It can also be used to reference a set of observation to a certain common time frame, such as the series of tide gauges being referenced to the same tidal datum epoch.

 

As Bill93 pointed out the North American tectonic plate moves about 1-2 cm per year (generally to the southwest). The national horizontal datum for the U.S., the North American Datum of 1983 (NAD 83) is defined as fixed and stable (generally) on the North American plate. In reality, there can be small motions within the plate. With contemporary GNSS technology these motions (velocities) can be detected and modeled. One of the primary elements of the recently completed national readjustment of NAD 83 was to assign each of the 69,000+ GPS stations in the network a common date of origin so that all positions can be related to the same reference frame. The date 2002.0 (January 1, 2002) was chosen because it is the epoch already used for the network of Continuously Operating Reference Stations (CORS), which were used as the control for this adjustment.

[foxtrot_xray]

[..]

One of the primary elements of the recently completed national readjustment of NAD 83 was to assign each of the 69,000+ GPS stations in the network a common date of origin so that all positions can be related to the same reference frame. The date 2002.0 (January 1, 2002) was chosen because it is the epoch already used for the network of Continuously Operating Reference Stations (CORS), which were used as the control for this adjustment.

So the EPOCH date on each benchmark (if it's listed) is the "date of origin" on the station's coordinates?

 

Like, say there's two stations nearby (~100ft). One has an EPOCH date of 2002.0, the other, say 2008.0. Technically, because of 'drift' and al the other fun stuff, the station with the EPOCH date of 2008.0 would be closer to it's coordinates? (less drift over a year than 7) (And yeah, I realize that the drift is so small that anyone would be hard-pressed to tell, but I'm just using it for example.)