Land Surveyor Community Forum

New ground control point

Good day to everyone

 my name is philos,

i want to establish 3 new control point. i have one reference point (know point) and the distance between all the points are less then 500 m.

can you please help me with a survey techniques (+ procedures) which can help to get a accurate points.

Note: i use a DGPS equipment, mark trimble

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  • Hi Philos,

    First question is about your equipment. Is it a DGPS or an RTK? DGPS is about one meter, half meter accurate, or if you have a better instrument, you may achieve 20 cm. What is the exact type of your GNSS receiver?

    If you have RTK (survey grade GNSS receiver and Real Time Kinematic corrections from a base station) you can reach 1-2 cm accuracy level.

    Second question is what do you want to do with the control point, and what is your expectations about the accuracy. Do you want to use that point as base for further GPS measurements or base point for total station measurements? What is the relative accuracy you want to reach (maximum error between your points)?

    Third question is the projection. Do you need projected coordinates, or geographical coordinates are enough? Have you got built in transformation, or you need to calculate the transformation parameters on the site?

    Generally we need more information about your project, your aims, your instrument, your map projection, the DGPS (or RTK) correction you can use, etc.

    • thanks you all for your message.

      Hi Peter,

      my equipment is a DGPS, those control point will be used for the aerial survey that why i need a high accuracy.

      i will need projected coordinated. after establishing those points i will do the site calibration to get the local site coordinate system.

      • Hi Philos,

        I've asked the type of your GPS because the answer DGPS is not detailed enough. Depending on the equipment, the DGPS message type, the distance from the DGPS base the accuracy can be 10 cm or 1 meter as well. Some of the DGPS capable GPS instruments can store raw GPS measurements and can processed later as post-processed differential GPS measurement, which is more accurate. Some instruments has dm capability, and it can reach 10 cm accuracy if real time DGPS beacon signal received from a base which is closer than 50 km.

        If you can log only real time positions, you can try to calculate a long time average from the positions, but this averaged coordinate will be only 2-3 times better than the original accuracy level of your instrument (I mean the estimated error will be around the half  or the one third of the single position error).

        So please accurately describe the type of your instrument (brand, type, version), the DGPS type you can receive (RTCM 1, 2, 3), and the distance between your points and the distance from the DGPS Base Station.

  • What I would do for the best result is set the base on one of them and localize the base to the known point. then shoot a second point RTK, and use that for a good azimuth. Set up an instrument and shoot between all three, horizontal distance and vertical difference, make those observations relative to the one known point and then localize on your three known values. Doing it this way about the easiest. And being within 500m your very safe in not having a big difference between grid and ground
  • Exactly what equipment are you using?
    • I have got a DGPS

    • knowing the coordinates of the ''known point'', you can use it as the base location and traverse on the other points using DGPS in static mode for better accuracy or in Real Time Kinematic (RTK) mode.

      • Hi abraham,

        if i use the static mode, for how long observation would i set at each point.

      • O.K., I don't want to steal your thread or anything. I know what DGPS is, I use it on my boat offshore. What coordinate base are you referring to? State Plane grid, rectilinear surface coordinates or absolute geodetic (latitude/longitude). Since you are using GPS, I assume you must be using a geodetic grid or latitude/longitude but with DGPS that is not necessarily 100% true. Am I wrong in thinking it could be a local, job specific grid? I am not an expert on this. I have learned a lot but I know just enough to get confused at times.

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