What unit does grn2eqa return?

Hello,

I have recently used the in build matlab function grn2eqa to convert a batch of lat-long data in degrees to equal area xy co-ordinates. However, looking at the results the unit does not make any sense to me (order *10^-5). The relative motions are quite small as this is gps data from a 40% scale model aircraft but i cannot figure out what unit this is or find anywhere that it is stated.

Does anybody know?

Cheers,

Louis

Answer 1

[Star Strider]

I don’t have the Mapping Toolbox, but a quick look at the online documentation for grn2eqa provides:

• [x,y] = grn2eqa(lat,lon,origin,units) specifies the units for the inputs, where units is any valid angle units string. The default value is 'degrees'.

[Louis]

I have been able to specify the unit of the input to the function, which in my case is degrees. My issue is I don't know what the unit of the output is.

[Star Strider]

Tracing back through the documentation:

• [x,y] = grn2eqa(lat,lon,origin,ellipsoid) specifies the ellipsoidal model of the figure of the Earth using ellipsoid. ellipsoid is a referenceSphere, referenceEllipsoid, or oblateSpheroid object, or a vector of the form [semimajor_axis eccentricity]. The ellipsoid is spherical by default.

This then leads to the documentation for Reference Sphere, particularly the information in:

• Examples
• > Create a Model of Earth in Kilometers

that reveals:

          Name: 'Earth'
          LengthUnit: 'meter'
          Radius: 6371000

leading me to believe that the default distance is meters.

[Louis]

Thanks! I hadn't realised what the ellipsoid argument of the function did so initially left it blank. I set up the reference sphere to the earth model provided in matlab and used this as the ellipsoid model in the grn2eqa function. This changed the values on my xy co-ordinates to meaningful co-ordinates in the unit that I specified the radius of the Earth in, in this case meters. Thanks again!

[Star Strider]

My pleasure!