Error: Array Out Of Bounds Error Details: Failed invoking <symbol> Help! No idea whats wrong.

Why don't you scale with scaleXY?

it should read

var pixelsLon = (lon[i+1] - lon[i]) * scaleXY;
var pixelsLat = (lat[i+1] - lat[i]) * scaleXY;

var lonMax = -180.0;
var lonMin =  180.0;
var latMax =  -90.0;
var latMin =   90.0;
for(i=0; i < lon.size(), i++) {
    lonMax = (lonMax > lon[i]) ? lonMax : lon[i];
    lonMin = (lonMin < lon[i]) ? lonMax : lon[i];
    latMax = (latMax > lat[i]) ? latMax : lat[i];
    latMin = (latMin < lat[i]) ? latMax : lat[i];
}

Why do you change the signs of the start values (lonMax, lonMin, latMax, latMin)?

For searching an array for a maximum value (e.g. lonMax) the variable should be initialized with a minimum value (e.g. lonMax = -180.0) and vice versa.

Sorry, I missed that bit 

Typing mistake, I make a lot of those.

Also been doing this in spare moments which are fewer at the mo so am rushing when I type.

copy/paste your code to avoid typos that waste the time of people trying to help and start using println calls to understand what is happening in your code.

I did copy and paste the code, the typing mistakes were done in eclipse. Some just weren’t enough to cause an error hence why they went unnoticed. I will try and remember to add printIn calls however.

hey sorry to annoy you with more questioning but i've now run into a different problem.

if(track != null){
var lonMax = -180.0;
var lonMin = 180.0;
var latMax = -90.0;
var latMin = 90.0;
for(var i=0; i < lat.size(); i++) {
    lonMax = (lonMax > lon[i]) ? lonMax : lon[i];
    lonMin = (lonMin < lon[i]) ? lonMax : lon[i];
    latMax = (latMax > lat[i]) ? latMax : lat[i];
    latMin = (latMin < lat[i]) ? latMax : lat[i];
    
    System.println("lonmax = " + lonMax);
    System.println("latmax = " + latMax);
    System.println("lonmin = " + lonMin);
    System.println("latmin = " + latMin);

}

var scaleX = dc.getWidth() / (lonMax - lonMin);
var scaleY = dc.getHeight() / (latMax - latMin);
var scaleXY = (scaleX < scaleY) ? scaleX : scaleY;
           
    System.println(scaleX);
    System.println(scaleY);
    System.println("scxy " + scaleXY);
            
for(var i=0; i < (lat.size() - 2); i+=2){

var pixelsLon = (lon[i+1] - lon[i]) * scaleXY;
var pixelsLat = (lat[i+1] - lat[i]) * scaleXY;
                
var displayX = pixelsLon + 120;
var displayY = pixelsLat + 120;
    System.println("draw locx = " + displayX);
    System.println("draw locy = " + displayY);
               
    dc.setColor(Graphics.COLOR_WHITE, Graphics.COLOR_TRANSPARENT);
    dc.fillCircle(displayX, displayY, 3);
    dc.setColor(Graphics.COLOR_BLACK, Graphics.COLOR_TRANSPARENT);
    dc.fillCircle(displayX, displayY, 2);
    }
}

This is the code i've got as a whole from the conversation, I am unsure if it is 100% correct as it's returning an invalid value on line 20 though has occured on line 19. I added println calls as Jim suggested and got this for the lon min/max/latmin/latmax calculations. I used a calculator and did the calculation in the code and some appear to return 0 which I think it dislikes, is that intentional? 

lonmax = 0.206338
latmax = 50.852655
lonmin = 0.206338
latmin = 50.852655

lonmax = 0.206338
latmax = 50.861510
lonmin = 0.201533
latmin = 50.861510

lonmax = 0.206338
latmax = 50.861510
lonmin = 0.067743
latmin = 50.830709

lonmax = 0.206338
latmax = 50.861510
lonmin = 0.206338
latmin = 50.861510

lonmax = 0.206338
latmax = 50.861510
lonmin = 0.194795
latmin = 50.856027

lonmax = 0.206338
latmax = 50.919657
lonmin = 0.109084
latmin = 50.919657

lonmax = 0.218890
latmax = 50.945271
lonmin = 0.218890
latmin = 50.945271

lonmax = 0.281672
latmax = 51.753031
lonmin = 0.281672
latmin = 51.753031

lonmax = 0.281672
latmax = 51.753777
lonmin = 0.163367
latmin = 51.753777

Also I am again unsure if the code I've used is the correct one for scaling the route to follow in storage to my map of europe:

It extends off screen (see below image) and changes as you move like a satnav (i've got a bit mixed up from all the examples you've kindly provided due to all my question asking). 

The map displayed on the watch, it's a bit faint as I need to modify it.

I hope this adequately explains everything.

Regarding my last issue, I figured out the problem, where it does the calculations:

lonMax = (lonMax > lon[i]) ? lonMax : lon[i];

lonMin = (lonMin < lon[i]) ? lonMax : lon[i];

latMax = (latMax > lon[i]) ? lonMax : lon[i];

lonMin = (lonMin < lon[i]) ? lonMax : lon[i];

(as well as the lat equivalent calculations)

I changed it to

lonMax = (lonMax > lon[i]) ? lonMax : lon[i];

lonMin = (lonMin < lon[i]) ? lonMin : lon[i];

latMax = (latMax > lon[i]) ? lonMax : lon[i];

lonMin = (lonMin < lon[i]) ? lonMin: lon[i];

I’m very grateful for all of your help with this.