L1 GPS for Urban Surveys..feedback please

> I want results of +/- 0.02' in 1200'. I have two stations to occupy in that length.
>
> Using L1 GPS, how long would I need to occupy each station to get the desired result? How long would it take to process L1 data to give me an answer?
>
> I was able to get my answer in 30 minutes using a TS while onsite.

Yeah, but you're the guy who then spent hours on this message board wondering whether he ought to upgrade his total station. If you recall, you wrote about that episode:

> Though my gun is specced at 5", it has proven itself to exceed that. Though I have not tested it to be sure, it probably does 3" work.
>
> Now...
>
> Having a 1" gun would be nice, but are the optics any different?
>
> I was shooting over 1200 feet today and had some nice overcast and cool temps but even so, it's pretty tough to get a real clear sight. If the optics were larger then the sighting would be much better.
>
> So..aside from the angular difference, would a 1" gun see a sight at 1200 feet any better than a 5" gun.
>
> I have a Pentax.

L1 GPS could have given an answer that would have freed you up to wonder about other stuff than whether your total station was giving you a bad answer or not.

30-45 Minutes For 0.01' Horizontal

I don't accept less. L1 is a bit more dependant on lots of satellites and constellation positions since it cannot fully account for atmospheric conditions.

Once set up, I can do something else in that 30-45 minutes. Usually I get so involved with something else I let them run. Even if you have to babysit the units, you pull out your laptop and work. Yesterday I downloaded my data collector, fit in a few points and then looked for and found an rebar with ribbon buried in a gravel road. It was a bit frustrating using the metal detector since it was in the area of an abandoned railroad crossing and I had my fill of digging out railroad spikes.

Paul in PA

No Kent

As usual, you read into a post things that are just not there.

I am happy with my Pentax and have no intention of upgrading. I asked a simple question about optics.

As far as the time I spend on this board well, it's my time and I do believe I can spend it any way I want. I have a lot of friends on this board Kent. Over the years I have tried to make you one but I see now that is just not going to happen.

Paul

> Once set up, I can do something else in that 30-45 minutes.

So that's 30-45 mins per station?

No Kent

Kent is your friend he's just that IJTF guy that likes to poke fun, in his own, dry, Kent sort of way.

Radar

Per Vector

If you only have one L1 receiver and are tying to CORS within 12 miles you are better off to do two 15 minute sessions on each point an hour or more apart.

Please explain your available equipment.

Paul in PA

paul

one caveat about single frequency. 30 to 45 would be good given you are fixing from a local monument. first session rec1 on local cmonu,rec2 on your new trav sta 'a'. session two;keep rec2 on sta'a', bring rec1 to sta 'b'. so,really, 30 to 45 for each session.
if you are fixing from cors, much longer sessions, dependent on baseline distance.

hope it helps.

< 20 minutes for +/-0.015 ft.

> I don't accept less. L1 is a bit more dependant on lots of satellites and constellation positions since it cannot fully account for atmospheric conditions.

You're right about the fact that on many projects time that GPS receivers are running can be spent doing something else productive. However, as to session lengths, using 20 minutes of observations under normally favorable conditions, my L1 Trimble 4600LS receivers will routinely give baselines with standard errors of +/-0.015 or less on the short lines (<1km) that are typical in city work.

Paul

If you have at least two receivers you can do it with a 15 minute observation assuming clear sky view. As you add obstructions such as trees (not under full canopy) or buildings, particularly blocking the southern sky then you need to add time up to an hour at most, generally.

It is likely you will get better than 0.02'.

I have three ProMark3s and they are perfect for this type of work.

I would be glad to come down there and do it for you on a Friday but you have to give me a place to sleep and something for my wife (the boss).

Paul

GPS is a measurement system just like your total station. It measures 3D courses and distances between receivers. Two receivers close together can measure a line in less time than two receivers far apart.

If you use one receiver and process to a CORS station 20 miles away then you have a radial survey with two vectors 20 miles long.

If you just want the 3D vector between two monuments 1200 feet apart then it's best to occupy them simultaneously with two receivers. I usually want to tie to CORS network so I use one receiver like a base station, let it run all day and the other two go around and measure the vectors I need plus they get vectors back to the base. I make triangles.

David

> I would be glad to come down there and do it for you ...

Thanks for the offer but all of the control measuring is done. Tomorrow I'll be monumenting the project. At first light we will be in the intersection. On a Sunday I can usually get two hours of very low traffic conditions even at the busiest intersections.

< 20 minutes for +/-0.015 ft.

True.

Typically in urban work the GPS receivers can only get a small sample of the points so they can be left running for longer periods of time while other tasks are accomplish.

I use them in concert with my total station to get a network of measurements in the most convenient fashion for me which typically means GPS receivers left to run longer than the minimum time. With GPS plus total station you think more in terms of the network and less in terms of the traditional closed traverse.

< 20 minutes for +/-0.015 ft.

I've truly learned something here today, would never even consider GPS, whether L1 or RTK, in a city environment.

Between the typically heavy tree canopy typical to South Alabama and multi story buildings, many times within a 50 foot right of way, what I see gives very limited sky view.

Kent, given the barren landscape where you typically work your faith is understandable and probably justified.

I've never worked urban California, so I have no relevance there, but given your state standards in order to get the warm fuzzies you must get good results or you wouldn't use it.

< 20 minutes for +/-0.015 ft.

> With GPS plus total station you think more in terms of the network and less in terms of the traditional closed traverse.

Yes, that's absolutely right. When the GPS vectors are independent redundant measurements in the network, you can start to assess real survey accuracy instead of having to rely upon very indirect QC measures like traverse closure.

< 20 minutes for +/-0.015 ft.

> I've truly learned something here today, would never even consider GPS, whether L1 or RTK, in a city environment.
>
> Between the typically heavy tree canopy typical to South Alabama and multi story buildings, many times within a 50 foot right of way, what I see gives very limited sky view.

Obviously, there are places where GPS will be a poor choice, but even in the central cities there usually are GPSable locations that can be used to excellent advantage once you get out of the urban canyons. As for tree-lined streets, I'd be surprised if even in that setting there aren't locations where breaks in the tree cover make GPS possible. T

< 20 minutes for +/-0.015 ft.

Don't think in terms of "I am going to take my network RTK rover and get all of the points."

Suppose you have a line of control points on a wooded ridge top. There's a hill top half a mile south which is devoid of trees. Put your base here. Let's say 4 of your 15 points on the ridge top are open sky enough (ideally you get a GPS point at either end of your line of points). Traverse along the ridge top through your 15 points. While you are traversing run your two other L1 receivers on two of the open points. Take a break every few hours and move receivers around. Your goal is to get a series of triangles hopefully with some reduntant vectors in there. One of the major advantages of GPS is that it can measure lines through hills, buildings and other obstructions. The only line of sight restriction is up to the sky. The two receivers don't have to see each other.

You can apply this same strategy in an urban context.

David

that's cool Paul.

GPS can be used to tremendous advantage but it's another tool that costs dollars to acquire so you have to evaluate if it would be profitable for you to do so. Sometimes it's just handy to be able to extend control say from street to street without having to go all the way around the long way with your total station. There's a thousand different scenarios I can probably think of. It mainly will strengthen your measured Survey figure.

GPS it's what's for Dinner.

David

>Sometimes it's just handy to be able to extend control say from street to street without having to go all the way around the long way with your total station.

I can relate to that David.

When was the last time you checked your GPS values with your TS? What was the variation in what distance?

David

I can't remember exact numbers except they were tiny.

Under 0.02' in 1200' is practically assured in good sky conditions. that's Static (15 minute observations minimum). I can do Stop&Go kinematic typically which are 15" observations and those will be rougher, more in the tenth range so I don't use it for control, just topo.

RTK is getting exponentially more popular every day but I'm not a big fan of the typical network RTK setup where you are supposed to be able to measure points using some vendor's base miles away. I use GPS not as a coordinate tool but as a measuring tool much the same as my total station. I don't have a big desire to get real time SPC coordinates on all my control points at super fast speed. Besides I don't want just a coordinate, I want vectors I can add to my local network.