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BackpacknJack

Suitable For Satellite Observation?

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Gathering info with two eyes but storing it in only half a brain has gotten me lost on this topic.

I remember seeing what is required to declare a BM as “suitable for satellite observation” (somewhere) and remember the sky line/horizon must be clear, except for maybe a power pole, at 15 degs and above and think I remember seeing that this site line starts 5ft above the mark.

( A ) All of the above is true.

( B ) Some of the above is true.

( C ) None of the above is true.

( D ) We're tired of answering this Q.

( E ) Stare at this little dot >>> . <<< until you see the answer.

( F ) Where in the world did you come up with....???? :(

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I gave up on being confident about knowing this long ago.

I always put either no or don't know since I don't hunt benchmarks in Nebraska, Kansas, or Oklahoma. :( There's always some tree or something that I can see from the mark. I think I've never really seen a mark that is suitable for satellite observation.

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I have run across blatantly satellite observable stations, like a disk along the east-west right of way fence on a highway, where it's flat prairie w/ nothing to the south. Otherwise, I usually put don't know.

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I design electronics for a living, and still can't reliably answer the question. IMO, no one really knows until they try. Some obstacle that looks like a disaster might have little impact, and something trivial could be a showstopper. I usually go with the line from the Red Green show, "those three little words men find so hard to say- I don't know!"

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In addition to the guidelines already mentioned, I consider how many satellites I am receiving and what the signal strength is. Also, if the unit won't lock in on a 7 to 9 foot accuracy indication, I consider reception poor.

 

Here are some where reception was good:

a00bf258-cbb8-4a8e-a257-49de3f876f38.jpg

 

1ffee128-f102-48fb-be80-d4bc98a90358.jpg

 

These were not suitable for GPS observations:

 

ed9641d7-e295-4567-b876-01fe51ac727c.jpg

 

19ac7413-90c6-4df2-9f35-32c8523f27cf.jpg

 

b9ce2739-2961-4ca6-817a-7e348ac56055.jpg

Signal reflections off stone chimney.

 

Everything in between usually is "Don't Know". :cute:

 

Paul

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The goal is to have unobstructed view of the sky at all times. If there are obstructions then siganl loss (slips) occur. So if you are observing for 5 hrs but have a a total of 1 hr of slips, you don't have the redundency of observations.

 

Think of it as shade. if the mark is in the shade a lot, NO GOOD. If a lone pole is present that is not likely to cast a lot of shade and not for long.. Just because the signal can come thru trees etc does not mean you are getting reliable signal.

 

With many GPS survey projects, they are observing muiltable stations simultanously so continunity of observations are very important and you do no want to have to throw out one station because it has a lot of slips and high DOP. I worked on the state of Mich HARN REO's in 2001 and we had many stations (dozens) + all the CORS going at the same time all over the state. We had people for many state agencies as well as private sector PS's wokring on these 5.5 hr sessions over 3 different days. I was doing just that on 9-11-2001..

Edited by Z15

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I thought I understood this until I read this thread. I thought the question was: could a satellite take a photograph of this station?

 

>Generally, a station is suitable for satellite observations

>if there is a clear and unobstructed view of the sky from

>approximately 15 degrees above the horizon at the location

>of the station. Small objects such as a light pole or

>small tree are excepted.

>

>Is this station suitable for satellite observations?

 

The term "clear and unobstructed view of the sky" implies visual observation. Am I off base here?

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68-eldo,

This thread refers to whether a professional grade GPS unit sitting on the mark (or 5 feet above it, or whatever) can observe (get line of sight radio signal) the GPS satellite constellation sufficiently to make good measurements. If you have stuff in the way, especially to the south, as the satellite orbital plane is south of zenith (directly overhead) for the continental US, then your measurement quality suffers. See Z15 and PFF's comments for the professional outlook.

Edited by BuckBrooke

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For professional work you want to collect date from all the birds that are possible, not just a few. Thus you near the "clear and unobstructed view of the sky". Meaning no trees 60 ft high 100 ft from the station, no houses or other buildings, hills, walls etc.

 

Unless you are out in the field doing the work it can be difficult to see how obstructions hinder final results. The better geometry from the birds to the rcvr, the better the results.

 

Here is an example of a perfect sky..it appears the trees are in the way but they are not as tall (10-15ft) as the photos leads you the beleive..

MVC-012X.JPG

 

The image below is the orbits of the sats at the given lat on the day of planned work. This is just some of the planning that goes into field work. The program allows you to plot obstructions and they determine the best times to observe to minimize slips. Its Leica software that only comes with thier GPS equipment..

The dark ring represents 15° above the horizon.

 

If you look at the image, the mark is dead center. Now plot the obstructions and you will see how the view closes up reducing the available birds to get data from and thus possibly weaking the geometric solutions.

 

gpssky.jpg

Edited by Z15

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The term 'suitable for satellite observations' can be a relative term. Sites which were marginally suitable (or questionable) years ago at the advent of GPS, may be suitable now due to newer technologies and more satellites. Some sites which were unsuitable, may be marginal, or even suitable now with a little bit of work. Not withstanding, a tree, building, or mountain will always be in the way if it's between the receiver and the satellite, no matter how good the technology is.

 

It has already been said that this term is intended for survey-grade GPS systems. This term does not apply to recreational grade GPSrs with a 10, or even 5 meter accuracy at best. The loss of a signal, whether momentarily or longer, from one or two satellites will not make much of a difference on the already large inherent error of the unit.

 

Most survey systems measure sub-centimeter, often in the 3-5 millimeter range. A signal loss there can wreak havoc on the data and the positional results. With older types of surveying (non-GPS), 'line of sight' was always critical. The instrument man needed to be able to see the rod man in order to take a measurement. That 'line of sight' is still important now, it's just that there are multiple rodmen (the satellites), and they're in the sky. Care must always be taken so the instrument man (the GPS receiver) can see them.

 

- Kewaneh

Edited by Kewaneh & Shark

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Someone (somewhere - can't find it at the moment!) actually used GPS signal reception as the basis for a "Landscape Picture". They set out a Survey Grade receiver for a good length of time (day+), and based on what sats they saw (or didn't) and when - they constructed a "picture" that showed the trees, house, etc as blank areas. It was interesting. Wish I could find it again.....

 

Edit: See link from Rotareneg a couple posts down. Thanks. Cool!

Edited by Klemmer & TeddyBearMama

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OK, if I were the NGS, I would put on the NGS website the following kind of guidance for answering their question about satellite suitability.

 

---------------------------

1. Hold a X-inch wide square upright at arm's length with the center of the square one foot higher than the level of your eyes. Examine the sky past the square at this upward angle while turning a full circle.

 

2. If an obstruction beyond the square looks twice as wide as the square looks at arm's length, count a score of 2 for that obstruction. Continue counting disks-worth of obstructions until you have turned a full circle.

 

3. If the total score is Y square's-worth of obstruction or more, the site is not good for satellite observation.

---------------------------

 

The basic idea here is that there would be a maximum of approximately Z scores per 360 degrees of skyline, each covering Z/360 degrees.

 

I think this kind of guidance would be far clearer than anything I've heard so far about how to determine the answer to this question. I realize that people have slightly different arm lengths and for some, and people have slightly different heights at eye-level, but I doubt this has to be really exact. As we realize, the only way to really tell is to set up a GPS station, etc.

Edited by Black Dog Trackers

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As a quick astronomer's rule of thumb :laughing: , your forefinger held at arm's length subtends (covers) about 1 degree wide. The width from thumbtip to pinkietip of your hand outstretched at arm's length is about 15 degrees, the requisite angle above the horizon.

Edited by BuckBrooke

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Someone (somewhere - can't find it at the moment!) actually used GPS signal reception as the basis for a "Landscape Picture". They set out a Survey Grade receiver for a good length of time (day+), and based on what sats they saw (or didn't) and when - they constructed a "picture" that showed the trees, house, etc as blank areas. It was interesting. Wish I could find it again.....

 

http://www.wsrcc.com/wolfgang/gps/panorama/

Edited by Rotareneg

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It would be nice if there was a simple formula to determine whether or not a site was observable, but there isn't. That determination involves more than just what is on the horizon, or just above it. Much of it is based on experience and professional site interpretation, proper planning prior to the site visit (using sky plots and DOP charts), and the ability to know the strengths and weaknesses of the equipment to be used in order to achieve the desired results.

 

Signal problems and interference are only one of the criteria used to determine site suitablilty. In addition to trees and buildings, problems can be caused by overhead and/or nearby power lines (usually well above the horizon), chain link fences, even large bodies of water (at, or below, the horizon). I once screwed up an occupation by parking my truck too close to the antenna, which was much higher than my truck.

 

The horizon at a given station can be a relative thing. The effective horizon for GPS station set-up is determined by the antenna height of the GPS station, not the eye, or shoulder height of the user. In my GPS work, I use a fixed height, 2.250 or 2.333 meter (about 7.5') tripod to set my station antenna on. If the station mark is elevated at all, the antenna is even higher, and consequently, so is the effective horizon.

 

There can also be physical obstacles which prevent the proper, or safe, set up of a tripod or other equipment that deem the site unsuitable. A station set on a steep hillside, on the edge of a building roof top, or a mark set on the top of headwall of a railroad trestle abutment can all make for precarious positioning. I know of one mark set in an airport runway apron. While it has a perfect view of the sky, it's also in line with the port-side landing gear of the incoming 767s and DC-10s. A technically suitable mark, but I wouldn't want to occupy it. (And hoping the airport would close runway two-niner left for a few hours is out of the question.)

 

It should be remembered that the term 'suitable for satellite observations' is not only a relative term (as mentioned in my post from yesterday), it is a technical term used by surveyors and engineers to describe a technical aspect of the marks. There is much more to it than counting nearby trees.

 

- Kewaneh

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Here is one we had a PITA with in 2001 but NGS insisted we had to occcupy it. It took 2 men.

 

 

QK0547 ***********************************************************************

QK0547 CBN - This is a Cooperative Base Network Control Station.

QK0547 TIDAL BM - This is a Tidal Bench Mark.

QK0547 DESIGNATION - 908 7096 TIE

QK0547 PID - QK0547

QK0547 STATE/COUNTY- MI/SCHOOLCRAFT

QK0547 USGS QUAD - HUGHES POINT (1972)

QK0547

z

z

z

z

z.

QK0547

QK0547 STATION RECOVERY (2001)

QK0547

QK0547'RECOVERY NOTE BY MICHIGAN DEPARTMENT OF TRANSPORTATION 2001 (GEK)

QK0547'STATION IS LOCATED ON THE PROPERTY OF MICHIGAN LIMESTONE OPERATIONS,

QK0547'INC., PERSONAL PROTECTIVE EQUIPMENT, AND ENTRY PERMIT REQUIRED (NOT

QK0547'SUPPLIED BY MLO, INC.). MARK IS DIFFICULT TO OCCUPY W/O SPECIAL

QK0547'EQUIPMENT, DUE TO A 3.5 FT HIGH WELDED STEEL RAILING 0.5 FT N OF THE

QK0547'STA., AND IS ON THE EDGE OF A BREAKWALL DROPPING OFF TO THE WATER.

1 National Geodetic Survey, Retrieval Date = FEBRUARY 24, 2006

Edited by Z15

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Kewaneh & Shark

Much of it is based on experience and professional site interpretation, proper planning prior to the site visit (using sky plots and DOP charts), and the ability to know the strengths and weaknesses of the equipment to be used in order to achieve the desired results.

So, I take it that your opinion is that unless we are a professional surveyor like yourself, we should always answer the NGS satellite suitability question (that appears at the last phase of logging at the NGS website) with I don't know.

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Kewaneh & Shark

Much of it is based on experience and professional site interpretation, proper planning prior to the site visit (using sky plots and DOP charts), and the ability to know the strengths and weaknesses of the equipment to be used in order to achieve the desired results.

So, I take it that your opinion is that unless we are a professional surveyor like yourself, we should always answer the NGS satellite suitability question (that appears at the last phase of logging at the NGS website) with I don't know.

 

I still think the question refers to the ability of a satellite to photograph the station (with appropriate marks to aid in identifying the station). When I suggested that earlier I was put in my place like I was butting in.

 

I sent the question to Deb Brown. The first message bounced so I just sent another. When I hear back from her I will post her reply here.

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To: 68-eldo:

 

I believe we all would agree that "suitable for satellite observation" could be taken two ways. Actually, I'd never thought about this until you mentioned it, but then I slapped my head and said, "Wow. He's right!"

 

In NGS terms, it refers to the ability of the GPS receiver to 'see' the satellites. However, you probably have seen the props used to make marks visible from the air. Often, it is a white paper arrow anchored to the ground, with the point exactly on the benchmark. These photos are taken by aircraft, flying over a specific project. However, I'm sure the modern satellites could "see" the marking, if somebody elected to zoom in.

 

Here's a sample. Look at the left center edge of the picture. I drew a box around the cross marking station WELCOME in Holly Springs, North Carolina. The scale of this photo is 1:3333, known officially as "Urban High Resolution".

 

Best regards,

Paul

 

48622e10-c71e-4ef3-88a7-464ce5f84563.jpg

 

Photo from TOPOZONE.

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68-Eldo: Without saying more than I am allowed, let me just say that IF there was the capability to photograph a benchmark disc from a satellite, DOD would hold that capability very close [internally] to DOD (ours or anyone else's), and it wouldn't be used for NGS survey purposes. Please don't believe the stuff you see in movies or on TV. Reality is sometimes disappointing. Sorry.

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Not to mention that no useful satellite reconnaissance could be done at an angle as low as 15 degrees.

 

Don't you just love the TV shows where "image enhancing software" has infinte zoom capability, and satellites can track the infrared signatures of people inside buildings through several floors?

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Kewaneh & Shark

Much of it is based on experience and professional site interpretation, proper planning prior to the site visit (using sky plots and DOP charts), and the ability to know the strengths and weaknesses of the equipment to be used in order to achieve the desired results.

So, I take it that your opinion is that unless we are a professional surveyor like yourself, we should always answer the NGS satellite suitability question (that appears at the last phase of logging at the NGS website) with I don't know.

 

BDT -

 

My explanations are not intended to be as extreme as 'If you're not a professional, don't make the suitable/unsuitable call'. I'm only trying to explain that there are times when making the determination as to whether a site is suitable or not is more complex than counting tall objects on the horizon. There are times when unsuitability is very obvious, like marks under heavy tree cover, marks placed in heavy urban or downtown area with many tall buildings, or the often mentioned indoor benchmarks. There are others which are obviously suitable, like many of those found on mountain tops, open plains, and the deserts of the southwest. In cases like these, (and many others, I'm sure), make the call.

 

My explanations are to express that caution be used when making a suitable/unsuitable determination, particularly if you plan on reporting the mark to the NGS. Because the NGS database is generally used by professionals for professional work, care should be taken when making a claim about a mark. This is similar to the declaration of mark being destroyed - destroyed on GC.com and at the NGS are different, and a mark declared destroyed by a recreational benchmark hunter may still be usable by a surveyor. Many times it's a simple call, other times, it's not so simple.

 

Use caution when making particular claims about a mark if you plan on logging it to the NGS. If you're sure it's suitable (or destroyed), report it as such; if you're unsure, write to the NGS and ask, log it as 'I don't know', or don't provide information that indicates an opinion either way.

 

- Kewaneh

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...In NGS terms, it refers to the ability of the GPS receiver to 'see' the satellites. However, you probably have seen the props used to make marks visible from the air. Often, it is a white paper arrow anchored to the ground, with the point exactly on the benchmark. These photos are taken by aircraft, flying over a specific project. However, I'm sure the modern satellites could "see" the marking, if somebody elected to zoom in....

 

Let's not confuse GPS measurements with photogrammetry.

 

GPS systems use radio waves of a known wavelengths to measure the distance from the satellite to a GPS receiver on the ground. Through triangulation, a position for the receiver can be calculated. The GPS satellites can't actually 'see' anything as they have no cameras. 'Hearing' is probably a better term as the GPS receivers actually listen for the signals from the satellites.

 

Aerial photogrammetry uses a camera to take a picture of the ground from (mostly) directly above. The aerial targets are used to show common points between images (many times the images are sequential), and to help the photogrammetrist 'ortho-rectify' the image. A surveyor provides x,y,z, coordinate information to the photogrammetrist to help them with stitching the images together for the mapping processes they perform.

 

- Kewaneh

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I will continue to use either:

"No"

or

"I don't know".

 

I'm only trying to explain that there are times when making the determination as to whether a site is suitable or not is more complex than counting tall objects on the horizon.
I accept that the determination is not just counting tall objects on the horizon, but also X. Until I know what X is, I cannot accurately answer "Yes" for any mark that is not either on top of a mountain with no manmade structures and above the treeline, or on a desert with nothing above 5 feet tall for a few miles.

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Perhaps I don't understand... Are you supposed to answer the question, or interpret the entry?

 

Suitable for satellite observations from the point of view of NGS or any surveyor is just a slightly subjective opinon on whether the site is 1) able to be occupied by a GPS receiver, 2) likely to be able to receive reasonably good data. That involves no nearby obstructions that might give interference to GPS signals or reflections giving multi-path, and no extensive overstory, i.e. trees, that will cause problems with GPS observations.

 

That is my take on it anyway, if you think you are to determine this factor, consider if the station is capable of being occupied with relatively open sky.

 

- jlw

 

I will continue to use either:

"No"

or

"I don't know".

 

I'm only trying to explain that there are times when making the determination as to whether a site is suitable or not is more complex than counting tall objects on the horizon.
I accept that the determination is not just counting tall objects on the horizon, but also X. Until I know what X is, I cannot accurately answer "Yes" for any mark that is not either on top of a mountain with no manmade structures and above the treeline, or on a desert with nothing above 5 feet tall for a few miles.

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Correct me if I'm wrong, but with the ability to use the Glonass constellation and the addition of new satellites to the Galileo constellation and a new frequency added to the L1 and L2 that is already available. A site that is not suitable for gps observation today could very well be suitable in a few years.

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No, that has nothing to do with it. Visibility is the concern. If obstructions are blocking the sky, nothing will help.

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No, that has nothing to do with it. Visibility is the concern. If obstructions are blocking the sky, nothing will help.

 

Which really does answer the question, sort of. There are times when I know that the station is suitable, and times when I know that it isn't. Around here, and in much of California, because of topography, the easy answer is often "No": too many hills, mountains, trees or buildings (or combinations of the above). In the Central Valley and Mojave Desert, however, the easy answer is usually "Yes", as the tallest thing around is a yucca or cotton plant. In the cases of those where a judgement call is beyond my expertise, I answer "No', under the theory that if it isn't obvious suitable, it isn't suitable. Besides which, if someone with more expertise comes along later and overrules me, I don't mind :-)

 

One thing that can help on occasion is to call up the datasheet, to see if a previous Recovery has already answered the question. I've seen this a half-dozen times, or so. Once a County Surveyor declared a station unsuitable for no reason that I could figure. There was nothing within a mile or more, and I had strong and steady signals on every possible bird. All I could figure was the guy/gal had never used a GPSr. But I left the previous judgement alone on my own Recovery, despite that it was several years later. If the Description doesn't jib with current physical conditions, I'll correct things, but some things I just won't, and this is one of them. Of course, mogle1's comments about new satellites and frequencies, and the fact that a Station that isn't suitable today could be in a few years might already have been applicable to the above Mark, but I didn't feel like taking that position. And in light of Z15's reply, I think I did the right thing.

 

By the way, Deb Brown answered some of this, in this forum, some time ago, but I can't find it. In any event, this has been a really informative discussion.

 

Jim

Edited by chaosmanor

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2. VISIBILITY - All horizontal and vertical stations selected must have adequate GPS

satellite visibility. The visibility should be minimally restricted from 15 degrees above

the horizon to the zenith, in all directions; see Attachment D for details. Minor

obstructions are acceptable, but must be depicted on the Visibility Obstruction

Diagram. For new stations, select a site relatively free of present and future anticipated

obstructions. Utility poles in the GPS field of view are tolerable, and they provide

security and a reference to help locate the mark. Set new marks at least 2 meters from

a pole, to the south if possible. Likewise, existing marks within 2 meters of a pole

should not be used. Marks should not be set or used if within 5 meters of a chain link

fence.

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To: 68-eldo:

 

I believe we all would agree that "suitable for satellite observation" could be taken two ways. Actually, I'd never thought about this until you mentioned it, but then I slapped my head and said, "Wow. He's right!"

 

In NGS terms, it refers to the ability of the GPS receiver to 'see' the satellites. However, you probably have seen the props used to make marks visible from the air. Often, it is a white paper arrow anchored to the ground, with the point exactly on the benchmark. These photos are taken by aircraft, flying over a specific project. However, I'm sure the modern satellites could "see" the marking, if somebody elected to zoom in.

 

Here's a sample. Look at the left center edge of the picture. I drew a box around the cross marking station WELCOME in Holly Springs, North Carolina. The scale of this photo is 1:3333, known officially as "Urban High Resolution".

 

Best regards,

Paul

Photo from TOPOZONE.

 

OK, here is Deb Browns answer and my email to her:

 

Hi Glen,...

 

Good question. The satellite question refers to the signal ability not photographs.

 

deb

 

Glen Houlton wrote:

 

> Mrs. Brown,

>

> My apologies if you have received this message a second time. I

> received a bounce message the first time I sent this, so I am trying

> again from a different email address. If you have already responded

> please disregard this message.

>

> As a geocher looking for and reporting benchmarks I find this entry on

> the reporting form.

>

> *Satellite Usage*

>

> Generally, a station is suitable for satellite observations if there

> is a clear and unobstructed view of the sky from approximately 15

> degrees above the horizon at the location of the station. Small

> objects such as a light pole or small tree are excepted.

>

> Is this station suitable for satellite observations?

>

> _ Yes _ No _ Don't know

>

> My assumption was that it referred to the ability of a satellite to

> photograph the benchmark (with appropriate markings to help locate the

> mark). However there is a discussion in the benchmark hunting forum

> about this. There they are talking about the quality of the GPS signal

> at the station’s location.

>

> So my question is: is this entry on the form in reference to visual

> observation by satellite or GPS reception from satellites? If it

> refers to visual I can look up and see if there are any obstructions,

> if it is signal quality I will need to mark them all as “I don’t know”.

>

> I want to make sure my reports are as accurate as possible.

>

> Thanks for your help.

>

> Glen Houlton

>

 

Thank you to PFF for at least seeing my point of view.

 

While driving I noticed a large X in the road and waypointed the spot with my GPSer. This X is obviously used for identifying a location in aerial photos. Unfortunately the Google maps satellite photo quality is not good enough to see that one.

 

But if you go to this photo you can clearly see the lines in the parking lot. These lines are not any larger than the ones on the X I waypointed. So logically I would assume that if you painted lines like this to mark the location of a benchmark you could see it in a satellite photo.

 

Oh, wait, TV is not real so I guess this is impossible. Google must have Photo shopped this in. Or Google has access to better satellite photos that NGS.

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Oh, wait, TV is not real so I guess this is impossible. Google must have Photo shopped this in. Or Google has access to better satellite photos that NGS.

 

Or Google is using aerial photos too, not just satellite imagery. That just happens to be the correct answer.

Edited by Rotareneg

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Aerial photography is routinely available at 1 foot or better resolution, which allows traffic lines to show up. The entire state of New Jersey is available at 1 foot resolution, and many other states offer similar aerial photos of selected regions. I'm sure the raw photos had considerably better resolution than that, it's just that 1 foot resolution makes was very convenient for orthophotos in state plane coordinates, so that's the resolution used for the digital images.

 

I find it interesting that Google does not in fact make use of all of the best imagery that is publicly available on the internet. It's often possible to find better imagery at state GIS clearinghouse web sites. Even venerable old Terraserver's 1 meter resolution USGS orthophotos are often better than Google's in some parts of the country.

 

The U.S. government for a llong time restricted access to satellite imagery that was better than several meters resolution. I believe the best current commercially available satellite imagery is about 1 meter resolution, from non-U.S. satellites. The U.S. remote sensing industry has complained loudly about the restrictions that prevent them from competing with foreign companies in high-resolution imagery. I seem to recall that even the non-U.S. companies generally refuse to release high-resolution satellite imagery of war zones.

 

edit: Here's a sample shot of the space shuttle Discovery take from the best available satellite imagery at about 2 foot resolution. Clearly it isn't enough to see a 4 inch bronze disk.

 

satellite_photo.png

Edited by holograph

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