Collecting Ground Control Points in the field.

Mr.FIlsonMr.FIlson Global Mapper UserPosts: 11Trusted User
edited February 2015 in Elevation Data
Hello all,

Im am trying to come up with a quick, easy, and inexpensive way of collecting my own (as accurate as possible) lat, long, & elevation ground control points in the filed.

I had heard that I could connect a GPS receiver to my laptop with Global Mapper on it and that GM would allow me to name and record the points that I want to record. I was told that using Global Mapper like this would also correct any errors with the GPS data and yield very accurate GPS points.

I simply need to get and record accurate points. I do not need to store, extorp, build a map, or do anything else with them.

I am manually entering them into my own aerial imagery by identifying the points visually in my images. I can even hand write them down on paper in the field for what I need them for if need be.

Any ideas, advice, or list of GPS units that will work for this and connected to my laptop would be greatly appreciated,

Thank you in advance for any help.

Comments

  • DC KelleyDC Kelley Global Mapper User Posts: 77Trusted User
    edited February 2014
    Mr.FIlson wrote: »
    I had heard that I could connect a GPS receiver to my laptop with Global Mapper on it and that GM would allow me to name and record the points that I want to record. I was told that using Global Mapper like this would also correct any errors with the GPS data and yield very accurate GPS points.

    Just a reply from someone whom is not a GM expert (but who is for hyper accurate GPS usage). There is nothing in GM that will "improve" the raw GPS data as such (other than post editing), but it can in fact plot accurate data to mm level when it is given it (thanks once again to Mike).
    Your typical low cost consumer GPS can yield about 3 meters or so with a common correctional source called SBAS. If that accuracy will suite your needs, quit reading. These lower costs devices can often be made to produce ~30cm accuracy with local corrections and moderate care in measuring, but when I say this, the cost to do it rises. If you want true RTK accuracy (<2cm to mm), and your are not prepared to learn a lot of GPS to save money, then the equipment will be rather costly at least for another five years or so.
    Define what 'accurate' means to you, and how long you can stay in one place to get it, and the price vs performance point can then be made more clear for your application. The word inexpensive in your post suggests to me you need to consider if a radius error of 3 meters under clear sky conditions can suit your needs.
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited February 2014
    Thank you for your comments DC, they are 100% correct. One thing I would add is to be sure to make sure whatever GPS unit you use can output its position to an externally connected computer. Many new units go into USB storage mode when connected and cannot provide NMEA position data. I would suggest a "puck" style GPS device designed specifically for use with a laptop, like a Garmin GPS 18, to ensure the easiest connection with Global Mapper.

    Thanks,

    Mike
    Global Mapper Guru
    geohelp@bluemarblegeo.com
    http://www.bluemarblegeo.com/
  • Mr.FIlsonMr.FIlson Global Mapper User Posts: 11Trusted User
    edited February 2014
    DC and Global,

    Thanks very much to the both of you for the info. I am looking for sub meter or better on accuracy. I would like to get 4 to 6" if possible.

    I tired connecting an old Trimble Geo XT unit to my laptop but that unit was so old, 2003, that it would not connect. I don't believe that the unit was even functioning correctly let alone able to connect to my modern HP laptop.

    My handheld Garmin Etrex did exactly what you said G_M, went into mass storage mode.

    The Garmin GPS 18 puck is only about 70 bucks which beats a modern day handheld from Trimble at 17k. I can get a puck overnighted to me to start working with as I need to get rolling on getting some ground control.

    Per DC Im wondering with a little elbow grease, further GPS knowledge, and some time invested in learning the ropes about post processing I could wind up with sub foot accuracy with this unit or maybe with something else in around the 1,000.00 range? I could possibly spring for a little more for something that would be better right right now.

    Thanks again for the help guys.
  • Mr.FIlsonMr.FIlson Global Mapper User Posts: 11Trusted User
    edited February 2014
    iSXBlue II GNSS | SxblueGPS

    Going to gives these guys a call in the morning to see where they are on cost. If you have time to check these receivers out Id be great full to see what possibilities there might be.
  • DC KelleyDC Kelley Global Mapper User Posts: 77Trusted User
    edited February 2014
    The theoretical limits for that type of receiver design is about 30cm and can not meet your stated design goal. Call 'em. Their marketing seem creditable to me, but I want to recall that they too (like many firm these days) has an "L1-RTK" solution, which in English means you can hold the few inches you seek. In general the term you will see for this is "decimeter accuracy" for such devices. Such things require current corrections in the feild so some form of radio or cell link is needed as well. Ask them.

    IF you can afford the delay to simply post process your field data to get the final value and then input to GM, then a couple of fairly simple methods can be used to gather the raw pseudo range code and carrier data on you laptop, along with the NMEA sentences you are now looking at. Once you get back to the office, you extract and convert this data to a format called RINEX and compute the final positions using such tools, and then input them into GM for you use. In various forms this is how it has worked for 30 years. By this method you are able to get RTK like results.

    If you look about there are any number of units below $300 bucks that can do this. Then you would likely be limited by your antenna choice (pucks are not good for sub meter use, a suitable ant is about 5~600 bucks). Of course I presume here your environment in not blocking signals etc. I will defer mentioning that you need to learn to plant a tripod and other servery details to learn about.

    Below is from a uBlox device showing what it gives with SBAS corrections only (about a meter and a half), I just kicked it one and let it run while I typed this. The antenna feed is from our firms reference station (in other words very costly and in a fixed know location) This device has not been told that it is holding still, which is a major advantage to the navigation filter to produce the accuracy you seek. If this same inexpensive unit was treated as I have described above, the walk observed would be about 5mm, a tiny dot in the center of the screen. Most L1 only GPS systems that can provide you code and carrier data can do similar results with some care, but you would now be walking along the road to understanding GPS details, not just simply using it.
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  • Mr.FIlsonMr.FIlson Global Mapper User Posts: 11Trusted User
    edited February 2014
    DC,

    Heres what I learned today:

    SxBlue receivers are from $2,100.00 to over $8,000.00.

    $2,100 version requires ArcPd and Ezsurv (another $1,900.00) and is not compatible with GM for field acquisition of data. It will however output after the post processing files that are compatible with and that can be used by GM.

    $8,000.00 unit will work with GM in the field (supposedly) and requires Omni Star. Supposed to seamless non-post processing solution to get <40cm.

    Im interest but at this point the uBlox looks to be a more affordable solution. Im in the proof of concept phase of what I am trying to do and think that a lessor investment right now is the way for me to go.

    I have some time and plenty of elbow grease so post processing, learning GPS, and planting a tripod will be fine. I do have some field skills and have already started putting together a very user friendly, portable, and rugged field rig for what ever I ultimately go with on a receiver and computer.

    Im on the uBlox site now trying to get acquainted with what they have. I need to learn exactly what I will need to have from them. I need to know if GM on my laptop will record the field data. I will also need to find out exactly what I will need in order to post process.


    Ive got a newer bluetooth HP laptop and newer bluetooth MacBook with Paralles. I have Windows on it.

    BlueMarble mentioned that several folks run their stuff on Macs this way but could not provide any support for it. I have not gone through the forums yet to find the Mac Heads out there running GM.

    I would like to be able to use my MacBook for field acquisition of the data only. Hopefully it will work for that.
    I could then do the post processing back in my office on a desktop PC or on my iMac, which ever is required.

    Can you help me out with a list of what I would need using uBlox?
  • Mr.FIlsonMr.FIlson Global Mapper User Posts: 11Trusted User
    edited February 2014
    I have Windows 7 on my Mac. Looks like everything that uBlox has is compatible with Win 7.

    I also have Windows 7 on my HP laptop.

    Thank you agin for all of the help!
  • DC KelleyDC Kelley Global Mapper User Posts: 77Trusted User
    edited February 2014
    Mr.FIlson:
    I hate recommend uBlox over others who are equally as adept at what follows but it is a true statement we do use uBlox with L1 RTK to perform 2cm accuracy in moving vehicles which for some periods of time matches what we get from our $20k GPS units. I suggest you check out an open source tool called RTKlib as a beginners way to see RTK working 'hands on' and then if that does not scarce you off this path (and you will see a number of other similar GPS units listed it can directly work with) then go ahead and get a uBlox device and their software tool. Putting that into the 'raw data' mode while you are in the field will give you data you can covert to RINEX and send to public services like the Canadian PPP service (Precise Point Positioning) or run the data through RTKlib tools and play with the controls. You can then take the decimeter results and place these into GM. Your term 'elbow grease' again comes to mind. While you are in the field with all this, use the NMEA 183 stuff to drive GM and make sure you are getting the data you expect to see.
  • Mr.FIlsonMr.FIlson Global Mapper User Posts: 11Trusted User
    edited February 2014
    Maybe your misunderstanding me. I do not want to get a PHD in GPS.


    I just need to find a GPS unit that is accurate and go to the field and get accurate ground control points. Thats it. I will keep searching for a GPS unit if no one here can suggest one.
  • Mr.FIlsonMr.FIlson Global Mapper User Posts: 11Trusted User
    edited February 2014
    DC,

    What to do you recommend? I thought that you recommended uBlox???
  • Mr.FIlsonMr.FIlson Global Mapper User Posts: 11Trusted User
    edited February 2014
    "If you look about there are any number of units below $300 bucks that can do this. Then you would likely be limited by your antenna choice (pucks are not good for sub meter use, a suitable ant is about 5~600 bucks). Of course I presume here your environment in not blocking signals etc. I will defer mentioning that you need to learn to plant a tripod and other servery details to learn about."

    Can you let me know what unit and antenna you are talking about? Would one of these units work with GM? What post processing is involved?

    I will be working in open areas and can use a tripod or range pole with bipod legs.

  • Mr.FIlsonMr.FIlson Global Mapper User Posts: 11Trusted User
    edited February 2014
    Found what I needed, Thanks very much for all of the information. :o
  • jmdg125jmdg125 Global Mapper User Posts: 1
    edited February 2015
    Hi Mr. Fllson - I have a very similar need and am wondering what method you decided to move forward with and what type of success you've had.
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