Since all my written fiction until now has been fairly short, I never developed a habit of keeping details notes on a map when writing. While one MIGHT get away with this on a created world, it's not so easy when writing a novel based on a world we know. During my final proof run of Year of the Dragon Lord I paid special attention to geography (location, distance, travel time, terrain, etc.) In the process, I found several gaffes, but I also found a huge inconsistency. I had Gerald and company doing the equivalent of Charlie Daniels going to L.A. via Omaha from somewhere in Deep South, USA. The printable online maps were fairly useless. After a couple of weeks of wishing Google maps allowed for annotation, it dawned on me that perhaps they did. A quick (Google, naturellement) search showed that it is not only possible, but simple!
The following was revised 2019/02/19 to reflect MyMaps having been split out of Google Maps. Steps
Enter mymaps.google.com in your browser's URL field.
Click [+ Create a New Map] . A map definition box appears.
Double click on Untitled Map and give it the title you want.
Go where you want to start and zoom in or out as needed.
Click the triangle by Base Map for a drop down to select the type of map you want to work with. I tend to switch back and forth between Satellite and Terrain.
Pick the mode you want to be in by selecting an icon under the search bar.
The default is Select items (grab/move). This not only lets you move the map as usual but lets you select items you have added (markers, lines).
When you select Add Marker you can add any number of markers like the ones Google drops when you search for a location. After you drop the marker a box appears. Change the name (unless you really want Point 1). You may also add additional text in the box provided. Save. You will be back in the default mode.
Draw Lines does just that. Click a spot to start. Move to the next spot and click, and so on. When you click twice on the same spot the line is finished and a box appears to let you name the lines and add other text. Hitting the Escape key while drawing lines undoes the entire current action on the line.
Between each set of points you drop for lines, a mid-point appears. You may drag this point anywhere to change the current line into two. To extend a line from an end point, move the end point to the new destination, and then drag the mid point to the next location. A new mid point appears between that and the end point; repeat as necessary.
The info box for lines includes a total distance for that line set.
The previous two modes also include icons to let you edit or delete the item.
Selecting a line or marker pops up the box with the definition for that item for editing.
Undo and Redo undo an entire action (marker or set of lines), or redo them.
You can also Add directions which I have not explored as the roads were a bit different 500 years ago.
The ruler lets you measure distances. Each time you click on the ruler the last measurement disappears.
To see the underlying map without the current layer, deselect the layer in the map definition box on the left.
You can add another layer by clicking on Add Layer.
The three dots to the right of Add Layer are the equivalent of a File menu for maps; use this for everything from changing maps to deleting maps.
Caveat If you do not have a supported version of a supported browser, you are hosed.