I have a custom world that I am trying to see square miles / kilometers on. From what I can tell on the grid function it is based on lat / long. Is there a way to convert between lat / long and miles? Or am I going about this the wrong way? My goal is to get an idea of how many square miles or kilometers an area of the map is. Yes, I can measure it with the distance tool, but I am hoping there is a little better / easier way to do that.
You can click around it, using the 'area' part of the Info menu.
Click on Info, then Area. Click around the map, edge or a continent or island etc., and when done, right click the mouse. A small popup will show that is the square miles or square kilometers. It depends on which template you use.
As for latitude and longitude, the distance between the longitude lines varies accoring to where on the globe you measure. The distance between longitudes on the equator will differ from the ones at say Alaska.
Square miles/kilometers are a function of area, which FT3 doesn't directly support.
The grids in FT3 are based on latitude and longitude, and their shape will vary by map projection. One degree of longitude (distance "left" or "right" on any of the basic cylindrical projections) for a spherical world is equal to worldCircumference/360*cos(latitude). At the equator (0 degrees latitude), a 40000 km circumference world has 40000/360*cos(0)=40000/360*1=111.111 kilometers for each degree of longitude. At 30 degrees of latitude, it's 40000/360*cos(30)=40000/360*0.866=96.225 kilometers. At 60 degrees of latitude, it's 40000/360*cos(60)=40000/360*0.5=55.556 kilometers. At 90 degrees, it's 40000/360*cos(90)=40000/360*0=0.000 kilometers (that's right, there's a singularity at the poles). This formula should look suspiciously like the formula for the Sinusoidal projection, btw.
http://edwilliams.org/avform.htm offers a useful set of equations for computing things on a round planet. Some of the formulae assume an ellipsoidal body and some assume Earth size, but most are general enough to be translated for an arbitrary planet without too much loss of precision. An important concept that may help is that there are 60 "nautical mile" elements per degree of latitude. For Earth, that works out pretty close to 1852 meters, which is the local defined value for "nautical mile".
By far the simplest way to get a good approximation for map size is to open the "View Properties" toolbar in FT3. It will give you the distance up and down as well as left and right on the centerline of your map. However, the accuracy of this information will vary wildly depending on map projection, center point, and zoom level. A slightly better way is to use the distance tool to get two perpendicular values and multiply them, but you will again have accuracy issues. FT3 doesn't have a tool for measuring area directly, unfortunately. The CC3+ measure area tool will give you area information, but it knows nothing about map projections. Unless you export a map with an equal-area projection, the CC3+ tool will also give you wildly inaccurate values.