How do I lay out my lines of latitude and longitude

[inspectorjavert]

So all of the guides to building a world climate seem to require having latitude and longitudinal lines. My problem is I"m having trouble finding a way to put them on my map as I'm not sure how to convert pixles, mm, or other measurement units used by inkscape into degrees of latitude and longitude. Should I just put an equator down then divide my page into 3rds and label each third 60 degrees, or there a less crude method of doing it?

[AzureWings]

The answer would depend on what projection you're using for working on the map. For the existing climate guides it can be convenient to create the map in an equirectangular projection, which although introducing worsening distortion near the poles does make placing lines of latitude and longitude easy - you can simply place the lines in terms of proportion of horizontal/vertical distance on the map and it works. So If your map was say 2400x1200 pixels, 180 total degrees (-90 to +90) latitude gives 1200/180 = 20/3 pixels per degree. While 20/3 isn't a nice whole number, in this case placing lines every say 30 degrees then means each line is separated by 200 pixels. Even if you can't line it up exactly, you can always take the pixels-per-degree value, multiply by the number of degrees you want to put a line at, and then round to the nearest pixel. One of the benefits of an equirectangular projection in this regard is that with the 2:1 ratio of horizontal/vertical dimensions the pixels-per-degree for longitude and latitude are the same.

If you aren't making your map in an equirectangular projection, the relationship between different longitudinal and latitudinal lines can become more mathematically complex and will depend on the projection you're using. The principle is the same - you can calculate the correct locations for each line (say, 30, 60 etc) you want to place mathematically by making a ratio between the proportional location of the lines within 180 or 360 total degrees and the proportional location in the pixel (or mm or whatever) dimensions of the image.