how to create a circle with the origin not at (0,0)

[jtc]

How far off are your radius measurements?  How do they compare to the GPS accuracy of your origin point and boom position?

-Its about 5 meters off. The GPS accuracy from the origin point and boom position do not exceed 1 meter. Its about +/- 0.5 to 1m.

If I look on the LV  XYGraph I made, its like there is an offset on the point of origin.

[jtc]

Thank you very much for this VI, Fitting on a Sphere, a very interesting function.

If i put more sets of X and Y value, the point of origin changes also. Likewise, the calculated Radius changes too, is this reliable? Is it better that I will input more X and Y values to be more accurate?

Is there a way to input the X and Y values automatically without having to input each value one at a time. Since I will have to do the inputting of values at the top of the crane, which makes it troublesome. Is  there a way while the boom is moving at max radius, it is recording X and Y values and automatically inputs to the fitting on a sphere function?

[jasonhill]

Is the boom arm chaning in length?  Is the center moving?  Is the boom level?  It just seems odd that the GPS measurements would be that far off.  When was the last time it was calibrated?  Can you calibrate a GPS unit?

I'm just saying that something seems rotten in Denmark.

[Ben]

If I missed this, please forgive...

You are converting Lat and Long to distance somewhere along the line I hope?

Ben

[jtc]

Is the boom arm chaning in length? 

- No.

Is the center moving? 

- No.

 Is the boom level? 

- I just found out that post is off level a bit. The foundation when it was laid out was not leveled.

 It just seems odd that the GPS measurements would be that far off.  When was the last time it was calibrated?  Can you calibrate a GPS unit?

- I dont know how to calibrate a GPS.

I'm just saying that something seems rotten in Denmark.

[jtc]

You are converting Lat and Long to distance somewhere along the line I hope?

- Kindly show me how can I do this please. Can you send me a sample VI?

[DavidJCrawford]

Hi

I think that Ben has given you a good clue. Its been LONG time since I have done any work with GPS. From memory you need to convert Lat/Long into Northings and Eastings used in the Universal Transverse Mercator Coordinate System. See http://en.wikipedia.org/wiki/Universal_Transverse_Mercator_coordinate_system

Also see here http://www.gps.gov.uk/guidec.pdf. This lets you see what the maths is like

Once you are in Northings and Eastings you will be in familiar territory using good old pythagoras to calculate distances etc.

good luck

David

[DavidJCrawford]

I couldn't edit the message I posted?? I found the master pdf http://www.gps.gov.uk/additionalInfo/images/A_guide_to_coord.pdf.

There could be errors in the coordinate transforms. This http://www.gps.gov.uk/additionalInfo/gpsSpreadsheet.asp explains that the error could be 5 meters for calculations based on GB datums.

David

[johnsold]

I do not have direct experience with GPS, but soem theoretical understanding and quite a bit of radio experience.

How many GPS receivers are you using? Is GPS subject to errors from multipath propagation? Your situation likely experiences multipath. Using a differential mode might be better. Receiver 1 is mounted on the ground outside the radius of the crane and becomes the refernce point. Receiver 2 is mounted at the top of the post (the point you called Xo, Yo). Receiver 3 is mounted at the end of the boom. Using simultaneous readings of all three receivers should allow you to detect and compensate for most errors. Another consideration would be to make sure that all receivers are using the same satellites for the calculations. If the receivers use different constellations of satellites the errors will likely be different.

Lynn

[jtc]

Thank you for the message Lynn.

That is true that my application is always subject to multipath especially on a crowded city which have lots of high rise buildings. I am using a GPS Receiver and a Beacon Receiver to get a pseudorange differential calculation. There is a fixed beacon reference station near the island that gives the reference point. The Beacon antenna is located anywhere near the Tower Crane. The serial output of beacon receiver goes to the gps receiver for differential calculation. The GPS Antenna is placed at the end of the boom, which always give the actual position. Like what David had sent, i am also using the same formulas and I am getting to two digits distance values.

My problem is , if i will plot all the points on a graph (circular path), it shows that the center is not at (0,0), but its either on the first or second or third or fourth quadrant depending on my location. I want to plot it by LabVIEW but I still do not know how.

The Fitting on a sphere function that was shared to me is a very interesting function. The more points I entered, the radius and point of origin value changes too. And I dont know if this is accurate enough for me.

I really apppreciate all your advises and thoughts and I am very thankful.