iOS Writing back to CLLocation object?

[xArtx]

Hi Guys,
Pretty much what the title says.
I can read from a CLLocation into a float value:

latitude = positionone.coordinate.latitude;

but the compiler doesn't want to know about writing back
to an object I declared myself:

positionone.coordinate.latitude = latitude;

I found that odd. Is there another way to do it?
Cheers, Art.

 

[robbieduncan]

CLLocation is an NSObject. CLLocationCoordinate2D is a struct. So this really represents why Apple choosing dot (.) for it's syntactic sugar was a bad idea as what is really being called here is:

[positionone setCoordinate.latitude:latitude];

as the first dot gets expanded/replaced with the accessor. It shoudl be clear that the above is absolute nonsense hence the compiler complaining. This should work:

CLLocationCoordinate2D coord = positionone.coordinate;
coord.latitude=latitude;
positionone.coordinate = coord;

 

[xArtx]

CLLocation is an NSObject. CLLocationCoordinate2D is a struct. So this really represents why Apple choosing dot (.) for it's syntactic sugar was a bad idea as what is really being called here is:

[positionone setCoordinate.latitude:latitude];

as the first dot gets expanded/replaced with the accessor. It shoudl be clear that the above is absolute nonsense hence the compiler complaining. This should work:

CLLocationCoordinate2D coord = positionone.coordinate;
coord.latitude=latitude;
positionone.coordinate = coord;

Thanks, Apple can have their two plus signs back, I really just want them
to calculate all my distances between arbitrary coordinates for me

Bah!
It won't allow the third line. This time it knows it's a read only property.
It might mean the harder way.... haven't implemented the great circle formula before, but i doesn't look too difficult.

 

[dejo]

...I really just want them
to calculate all my distances between arbitrary coordinates for me

Have you looked into CLLocation's distanceFromLocation: instance method?

 

[xArtx]

Have you looked into CLLocation's distanceFromLocation: instance method?

That's what I wanted to use, but you have to be able to write coordinates to
a CLLocation object first (in order to calc the distance between arbitrary points).
It's a cinch if you are using it between points the device has actually been to.

Didn't take long to find a drop in solution (in Km)

    //calculate haversine distance for linear distance Km
    double lat1 = screenbrlat;
    double long1 = screentllon;
    double lat2 = screenbrlat;
    double long2 = screenbrlon;
    double dlong = (long2 - long1) * d2r;
    double dlat = (lat2 - lat1) * d2r;
    double a = pow(sin(dlat/2.0), 2) + cos(lat1*d2r) * cos(lat2*d2r) * pow(sin(dlong/2.0), 2);
    double c = 2 * atan2(sqrt(a), sqrt(1-a));
    double d = 6367 * c;
    float distance = d;

Edit,,, In this case, instead of drawing the usual ruler to a GPS screen,
I want to always display the total width of the screen in Kilometers.
The program already knows the real geographic coordinates of each corner of the screen.

 

[dejo]

Here's a quick version using CLLocation:

CLLocation *oldLocation = [[CLLocation alloc] initWithLatitude:lat1 longitude:long1];
CLLocation *newLocation = [[CLLocation alloc] initWithLatitude:lat2 longitude:long2];;
CLLocationDistance meters = [newLocation distanceFromLocation:oldLocation];

 

[xArtx]

Lol.. Bummer ...I might as well check for consistency!

I just learned that the ruler at the bottom of the screen is rather useless
if the view shows a whole continent.
As long as the grid is square, the distance measured across the screen
at one point on the y axis is not going to be the same on another.

 

[dejo]

...I might as well check for consistency!

Not sure what you mean by this.

 

[xArtx]

Not sure what you mean by this.

Give me a sec...

 

[xArtx]

Sorry, internet is almost back to dial up for exceeding bandwidth.
Anyway,

I might as well check for consistency!

I totally got that answer wrong...
I might as well check that the output of the C routine posted above
matches the output of the native code you posted.

 

[dejo]

I might as well check that the output of the C routine posted above
matches the output of the native code you posted.

Understood. Now I'm curious: If you find the output doesn't match, which one are you going to trust and how would you decide that?

 

[xArtx]

Measure the distance of one degree on a grid.
We can get a mean radius from a mean circumference which is defined in the formula.

----------

The formula doesnt account for the Earth's slight eliptiod (I don't think). Now I'm no longer thinking
I can just measure a degree longitude at 45deg South, and multiply it to see if the mean circumference matches.

----------

The formula doesnt account for the Earth's slight eliptiod (I don't think) But the defined mean circumference might. Now I'm no longer thinking
I can just measure a degree longitude at 45deg South, and multiply it to see if the mean circumference matches.

 

[xArtx]

Hey, I followed through

Not sure I explained myself well there.
In a nutshell, measure what each formula thinks the circumference of
the Earth is at a given degree of latitude, and compare with a known one
(doing it by measuring one grid and multiplying it).

I think I'd rather just keep the C one because I can access every component of it.
If I move to Mars for example, there's only one small change to make,
and don't have to wait for an iOS update.

They do disagree slightly, but I do think that's due to a different definition
of mean circumference of the Earth.
If you wanted to be anal, you could increment it in the C version
to see if it ever agreed all of the time.

Measurement across the screen for the picture posted above,
Australia must be in a different lat postion on the screen,
this time it's 4320.7Km across the middle versus the native code 4333.9Km.