5 ms·

"The Sentinel-1 satellites have shown that the Millennium Tower skyscraper in the centre of San Francisco is sinking by a few centimetres a year" I'm no engine

"The Sentinel-1 satellites have shown that the Millennium Tower skyscraper in the centre of San Francisco is sinking by a few centimetres a year" I'm no engineer - but to me "a few cm a year" seems very significant, and would be easily detectable by current surveying technology? You would probably even visually be able to see it at the entrance to the building after 1-2 years? Why was a satellitie needed here to confirm it?

eitally 10y ago
It wasn't, but if you have a satellite, why not? :) http://www.nytimes.com/2016/11/04/us/san-francisco-files-lawsuit-against-sinking-millennium-tower.html?_r=0 http://www.nytimes.com/2016/11/04/us/san-francisco-files-law... http://sf.curbed.com/2016/9/16/12945600/why-millennium-tower-sinking http://sf.curbed.com/2016/9/16/12945600/why-millennium-tower... http://www.nbcbayarea.com/news/local/San-Francisco-Taxpayers-on-the-Hook-For-Millennium-Tower-Sinking-Documents-Suggest-400098591.html http://www.nbcbayarea.com/news/local/San-Francisco-Taxpayers...
tajen 10y ago
I've learnt from previous discussions on HN that it is indeed a major source of concern for everyone involved, especially since it's also tilting and since it's a seismic area. They build the sidewalks again a few times a year. The tower is quite new and the builders are probably liable. The use of the satellite is probably rather to highlight that the Millenium tower isn't the only one.
Dylan16807 10y ago
A cheap GPS device can give you millimeter-level measurements every few hours, so using an active satellite does seem like overkill.
- clord 10y ago
  The article points out this can cover huge swaths and millions of buildings at once. It speaks of covering a continent with subsidence alerts. Afixing gps to each building might not be competitive.
  - Dylan16807 10y ago
    It can do that. And that's pretty cool. But that's not what they chose to focus the article on.
    - privong 10y ago
      > But that's not what they chose to focus the article on. The first few paragraphs are about the Millennium Tower, but that's just the hook. The rest of the article discusses the wide-area applications, including SF, elsewhere in the Bay Area, and Oslo, Norway.
- surveyor 10y ago
  I don't know what your exact definitions is of "cheap", "millimeter-level", and "every few hours", but your comment does sound a bit misinformed. What you are possibly referring to is called dimensional control in geomatics and civil engineering, and there are various measurement techniques that can be applied, including GNSS-based. In a way, the hardest part of it is the planning phase, where you have to decide which technique is most appropriate to meet precision, accuracy, timeliness, and cost constraints. All those factors kind of fight against each other. Finding the sweet spot is non-trivial.
  - Dylan16807 10y ago
    Okay, tell me where I'm wrong: AFAIK you can buy a few hundred to few thousand dollar receiver that tracks the carrier phase, let it sit around for a few hours, and then with aggressive post-processing you should be able to figure out the right phase and know your location within a few mm. Edit: And worst-case, you should be able to do an exact survey once, and then you can remember that and know which phase to use in the future. (Barring sudden multi-cm shifts, but in that case you have bigger problems.) You still have atmospheric fluctuations to deal with, but that's why you're measuring all day.
    - 7952 10y ago
      Vertical accuracy is usually lower than horiztonal accuracy. It is more difficult to calculate elevation than lat/long.
      - Dylan16807 10y ago
        Not by a huge amount. Should be less than 2x, at least at some point during the day.
        jofer 10y ago
        Much worse that 2x. Closer to an order of magnitude worse.
- jofer 10y ago
  Actually, it doesn't. The vertical precision of GPS is quite low, so measuring subsidence with gps is very difficult.
- 7952 10y ago
  This satellite produces archive data on a continuous basis. The entire point is that it is cheaper than sending someone with a GPS!
- mturmon 10y ago
  I wanted to point out that, as you mention, with aggressive post-processing, you can routinely get mm vertical accuracy from GPS measurements. This has been done for several decades now, but it's not an off the shelf result. (Not a "cheap GPS device") The post processing technology was pioneered by NASA among others. In the western US there is a network of over 1000 such monitors, anchored very deeply, that use these methods. Here is the time series from one in the Bay Area: http://www.unavco.org/instrumentation/networks/status/pbo/overview/P176 http://www.unavco.org/instrumentation/networks/status/pbo/ov... In that plot, you can see lots of interesting effects. First, the virtually continuous lat/lon velocity. Second, the annual vertical trend which usually has to do with groundwater. Additionally, smaller and larger perturbations from all kinds of sources from nearby construction to large scale seismic effects like distant earthquakes. GPS of this kind is a complementary technology to the fantastic InSAR measurements featured in TFA. Both are widely used in geodesy to measure seismic deformations and deformations due to groundwater extraction. The two approaches offer different temporal, spatial, and accuracy trade offs. For a building like this, if you really wanted to measure subsidence, GPS would yield more temporal information than InSAR. If I recall correctly, the fundamental limit to the GPS accuracy is uncertainty on tropospheric propagation of the radio signal. I don't know if this technology is readily available for one-off commercial applications, but it wouldn't be surprising. The required accuracy is clearly within reach, and the software to do the post processing is widely licensed (https://gipsy-oasis.jpl.nasa.gov https://gipsy-oasis.jpl.nasa.gov).
jdehls 10y ago
You can certainly measure this with current surveying techniques. What the satellite InSAR technology allows is the measurement of millions of points at the same time, over thousands of square kilometres. And with the Sentinel-1 satellites providing free images every 6 or 12 days, the measurements can be updated frequently.
- notahacker 10y ago
  Authoritative third party records that establish the period over which a building or land started to move is also potentially useful in resolving disputes over who and what might be responsible for the movement (nb: I'm currently working with startup that consults on satellite/legal issues)
- Animats 10y ago
  You can certainly measure this with current surveying techniques. Not easily. And compared to what? The classic USGS survey markers don't have their height measured accurately enough, and they move with the land. The USGS does have precision "vertical control point" markers, and there are about ten of them along the northeastern coast of San Francisco. The highest precision vertical control point nearby is a GPS station on top of Building 1 at Fort Mason, and its height is valid only to 1.35cm accuracy at 95% confidence.[2] Absolute elevation is tough to measure at those scales. Relative elevation, relative to the average of all ground points for, say, a kilometer radius, is something a radar satellite can do well. [1] https://www.ngs.noaa.gov/NGSDataExplorer/ https://www.ngs.noaa.gov/NGSDataExplorer/ [2] http://www.ngs.noaa.gov/cgi-bin/ds_mark.prl?PidBox=AF8575 http://www.ngs.noaa.gov/cgi-bin/ds_mark.prl?PidBox=AF8575

eitally 10y ago

It wasn't, but if you have a satellite, why not? :) http://www.nytimes.com/2016/11/04/us/san-francisco-files-lawsuit-against-sinking-millennium-tower.html?_r=0 http://www.nytimes.com/2016/11/04/us/san-francisco-files-law... http://sf.curbed.com/2016/9/16/12945600/why-millennium-tower-sinking http://sf.curbed.com/2016/9/16/12945600/why-millennium-tower... http://www.nbcbayarea.com/news/local/San-Francisco-Taxpayers-on-the-Hook-For-Millennium-Tower-Sinking-Documents-Suggest-400098591.html http://www.nbcbayarea.com/news/local/San-Francisco-Taxpayers...

tajen 10y ago

I've learnt from previous discussions on HN that it is indeed a major source of concern for everyone involved, especially since it's also tilting and since it's a seismic area. They build the sidewalks again a few times a year. The tower is quite new and the builders are probably liable. The use of the satellite is probably rather to highlight that the Millenium tower isn't the only one.

Dylan16807 10y ago

A cheap GPS device can give you millimeter-level measurements every few hours, so using an active satellite does seem like overkill.

clord 10y ago
The article points out this can cover huge swaths and millions of buildings at once. It speaks of covering a continent with subsidence alerts. Afixing gps to each building might not be competitive.
- Dylan16807 10y ago
  It can do that. And that's pretty cool. But that's not what they chose to focus the article on.
  - privong 10y ago
    > But that's not what they chose to focus the article on. The first few paragraphs are about the Millennium Tower, but that's just the hook. The rest of the article discusses the wide-area applications, including SF, elsewhere in the Bay Area, and Oslo, Norway.
surveyor 10y ago
I don't know what your exact definitions is of "cheap", "millimeter-level", and "every few hours", but your comment does sound a bit misinformed. What you are possibly referring to is called dimensional control in geomatics and civil engineering, and there are various measurement techniques that can be applied, including GNSS-based. In a way, the hardest part of it is the planning phase, where you have to decide which technique is most appropriate to meet precision, accuracy, timeliness, and cost constraints. All those factors kind of fight against each other. Finding the sweet spot is non-trivial.
- Dylan16807 10y ago
  Okay, tell me where I'm wrong: AFAIK you can buy a few hundred to few thousand dollar receiver that tracks the carrier phase, let it sit around for a few hours, and then with aggressive post-processing you should be able to figure out the right phase and know your location within a few mm. Edit: And worst-case, you should be able to do an exact survey once, and then you can remember that and know which phase to use in the future. (Barring sudden multi-cm shifts, but in that case you have bigger problems.) You still have atmospheric fluctuations to deal with, but that's why you're measuring all day.
  - 7952 10y ago
    Vertical accuracy is usually lower than horiztonal accuracy. It is more difficult to calculate elevation than lat/long.
    - Dylan16807 10y ago
      Not by a huge amount. Should be less than 2x, at least at some point during the day.
      - jofer 10y ago
        Much worse that 2x. Closer to an order of magnitude worse.
jofer 10y ago
Actually, it doesn't. The vertical precision of GPS is quite low, so measuring subsidence with gps is very difficult.
7952 10y ago
This satellite produces archive data on a continuous basis. The entire point is that it is cheaper than sending someone with a GPS!
mturmon 10y ago
I wanted to point out that, as you mention, with aggressive post-processing, you can routinely get mm vertical accuracy from GPS measurements. This has been done for several decades now, but it's not an off the shelf result. (Not a "cheap GPS device") The post processing technology was pioneered by NASA among others. In the western US there is a network of over 1000 such monitors, anchored very deeply, that use these methods. Here is the time series from one in the Bay Area: http://www.unavco.org/instrumentation/networks/status/pbo/overview/P176 http://www.unavco.org/instrumentation/networks/status/pbo/ov... In that plot, you can see lots of interesting effects. First, the virtually continuous lat/lon velocity. Second, the annual vertical trend which usually has to do with groundwater. Additionally, smaller and larger perturbations from all kinds of sources from nearby construction to large scale seismic effects like distant earthquakes. GPS of this kind is a complementary technology to the fantastic InSAR measurements featured in TFA. Both are widely used in geodesy to measure seismic deformations and deformations due to groundwater extraction. The two approaches offer different temporal, spatial, and accuracy trade offs. For a building like this, if you really wanted to measure subsidence, GPS would yield more temporal information than InSAR. If I recall correctly, the fundamental limit to the GPS accuracy is uncertainty on tropospheric propagation of the radio signal. I don't know if this technology is readily available for one-off commercial applications, but it wouldn't be surprising. The required accuracy is clearly within reach, and the software to do the post processing is widely licensed (https://gipsy-oasis.jpl.nasa.gov https://gipsy-oasis.jpl.nasa.gov).

jdehls 10y ago

You can certainly measure this with current surveying techniques. What the satellite InSAR technology allows is the measurement of millions of points at the same time, over thousands of square kilometres. And with the Sentinel-1 satellites providing free images every 6 or 12 days, the measurements can be updated frequently.

notahacker 10y ago
Authoritative third party records that establish the period over which a building or land started to move is also potentially useful in resolving disputes over who and what might be responsible for the movement (nb: I'm currently working with startup that consults on satellite/legal issues)
Animats 10y ago
You can certainly measure this with current surveying techniques. Not easily. And compared to what? The classic USGS survey markers don't have their height measured accurately enough, and they move with the land. The USGS does have precision "vertical control point" markers, and there are about ten of them along the northeastern coast of San Francisco. The highest precision vertical control point nearby is a GPS station on top of Building 1 at Fort Mason, and its height is valid only to 1.35cm accuracy at 95% confidence.[2] Absolute elevation is tough to measure at those scales. Relative elevation, relative to the average of all ground points for, say, a kilometer radius, is something a radar satellite can do well. [1] https://www.ngs.noaa.gov/NGSDataExplorer/ https://www.ngs.noaa.gov/NGSDataExplorer/ [2] http://www.ngs.noaa.gov/cgi-bin/ds_mark.prl?PidBox=AF8575 http://www.ngs.noaa.gov/cgi-bin/ds_mark.prl?PidBox=AF8575