Hello Brian,

Can you explain how are the coordination translated into UTM grid and also the convergence angle?

What are the parameters " E: and N: " (start point) for?

Latitude and Longitude are converted to UTM. The number are meters from the equator and around the planet from 0 longitude so are quite huge, you see those when a field isnâ€™t open. When you open a new field you may be in zone 12 and 612,765.43 north of the equator. So the offset 612,765 is subtracted and stored for that field. That is the utm offset for the northing value, same is calculated for the easting value. think of them as x and y on a cartesian grid. So the E: and N: just show the residual of the actual utm coordinate minus the offsets.

Universal Transverse Mercator coordinate system - Wikipedia

Also convergence angle is also calculated and that point is rotated in world reference because we live on a football and its impossible to have a square gride on an oblate spheroid. UTM Declination - YouTube

n position designer.cs line 1031:

`Preformatted text`

```
//reduce the huge utm coordinates
pn.utmEast = (int)(pn.fix.easting);
pn.utmNorth = (int)(pn.fix.northing);
pn.fix.easting = pn.fix.easting - pn.utmEast;
pn.fix.northing = pn.fix.northing - pn.utmNorth;
//calculate the central meridian of current zone
pn.centralMeridian = -177 + ((pn.zone - 1) * 6);
//Azimuth Error - utm declination
pn.convergenceAngle = Math.Atan(Math.Sin(glm.toRadians(pn.latitude)) * Math.Tan(glm.toRadians(pn.longitude - pn.centralMeridian)));
lblConvergenceAngle.Text = Math.Round(glm.toDegrees(pn.convergenceAngle), 2).ToString();
//Draw a grid once we know where in the world we are.
isFirstFixPositionSet = true;
worldGrid.CreateWorldGrid(pn.fix.northing, pn.fix.easting);
```

Brian,

Utm versus wgs84

the Gps system comes with wgs84 projection and the wgs84 geoid

Utm is a very similar ,very close projection and eventually very easy to translate from wgs84 :one single addition

That is why you think you use utm; the Nmea sentence is wgs, unless the config file is modified

I did the mistake until I used a Gis like Grass or Qgis :my fields where in the pacific ocean

No reason to be worried unless you use Landsat images for remote sensing

Python is fantastic ! sorry I did not study it in my young years

best regards

Are you thinking i am using wgs internally in the program rather then utm?

I should be carefull : as long as I haven t seen your calculus /and the Nmea sentence I cannot be sure

But the gps system is absolutely linked with the pair wgs84/wgs84 geoid

as you say you canâ€™t project precisely on a flat map without â€śdeformationâ€ť

to be simple some projections â€śbetrayâ€ť the surfaces ,some others betray the angles

By chance utm and wgs are so close each-other that you wonâ€™t see much : your region is utm12

I can explain some basics:

if you are in the middle of the region 12 , you must be an expert to detect any difference

but if you are on the edges of the region it can be 100 m offset

Today I can say that you should look at the Nmea sentence and be sure you do not translate it to Utm

I can help : one X and one Y of a crossroad (max decimals), plus a map :Gmap is wgs84

send to me ,I can have a look

I am very sure we translate to utm. The entire program uses it. Outer fringe error is less then 0.04%. Acceptable enough. We arenâ€™t doing seismic lines or surveying with AOG