Making a GPS to vector app, how to scale the height?

General
1

Hi, I’m making an app for taking a GPX (a list of GPS) coordinates and converting it to a vector that can be read by apple motion, so that I can animate and design my own path.

The altitude is done, and now the path on where is next up.

The vector is done, but I can not figure out how to scale it properly. My map, that has a lat of 46.43, but how to scale this, I now it is suppose to be around 1.5, but how to get to it and how to calculate it…

I read a lot about a Mercator_projection, but how to get this into xojo?

I have the lat and lon, in degrees, so I have the max and min of both lon and lat.

In my head I should be able to get a ratio of the map in a one to one by these numbers. But then I have to scale it even further based on where on the earth I am. So according to wikipedia :

at latitude 30° the scale factor is k = sec 30° = 1.15,

at latitude 45° the scale factor is k = sec 45° = 1.41,

at latitude 60° the scale factor is k = sec 60° = 2,

at latitude 80° the scale factor is k = sec 80° = 5.76,

at latitude 85° the scale factor is k = sec 85° = 11.5

but how can I make a formula to get these values…

any thoughts?

2

The secant of an angle is the inverse of the cosine, so k = 1/cos(latitude). Take into account Xojo’s cosine works with radians.

Is that what you meant? Otherwise, I didn’t understand your question.

Julen

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3

By radians you mean ? Not my strongest area… :slight_smile:
But I have the latitude in degrees… only. so a middle point here from the max to min would give me a good point, but from this I would use that altitude and figure out what the scaling should be. Right ?

4

Radians is another unit for measuring angles: 180 ° = pi radians (where pi equals 3.1415…).

In xojo you should calculate the sec of your latitude as:
K = Sec latitude = 1/cos (latitude*pi/180)

I don’t understand what your problem is, probably becaude I have never worked with GPS data, so I can’t help you further unless you explain the problem in more detail or point me to some reading that would help me understand it.

Julen

5

Hi Julen, and thanks for trying to help me. The sec, rad etc. I don’t know much about it…

But from the list in the first post, I need to get a coordinate around 60° to give me a plain 2 in scalefactor of y vs x.

I map all the vectors like 1:1 in my map, but since there is happening scaling vertical, I need to stretch the Y accordingly to this formula…

6

Did you try what I posted above? Any problem with it?

7

Hi Julen, yes, this looks promising. I just have to try it in my app…

8

YESSSSS! looks like this will do the trick… Thanks in a millions :slight_smile:

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