- Sketch your lines lightly in pencil at first. Everyone makes mistakes, so you'll need to erase frequently.
- You can start anywhere on the map, but try starting at the edges of the data.
- Think of the isoline as a fence, with lower values on one side and higher values on the other side.
- Go slowly and don't look too far ahead. Concentrate on the numbers in the area in which you are working. For example, a 1000mb line in New York isn't always connected to the 1000mb line in California. They may be two different systems.
- Lines of different values don't cross. A 50F line cannot cross a 40F line - because it can't be both 50F and 40F at one time at one location - but they can come close.
- Lines are either open or closed. An open line starts at an edge of the data and goes to another or the same edge. An isoline which does not extend to the edge of the data must be closed; it will loop back on itself, forming a closed shape.
- Open lines don't just stop in the middle of the map, there's always a way out. You need to look carefully.
- Open lines don't become closed lines and vice versa.
- Sometimes there is an odd reading that just doesn't fit well. This is often due to the elevation of the station, or sometimes there's an erroneous value. Ignore the reading.
Edited and adapted from http://fsc.fernbank.edu/MetSeis/wxmapactiv.html