This isn’t as straightforward as it initially appears. If you need to digitize some points of interest, you create an empty point layer and start clicking away. If you need to digitize some river or road lines, you create an empty line layer and start tracing. But if you follow the same logic to create polygons, unless those polygons don’t touch, you’re going to run into trouble.

Polygons that are separate from one another, such as building footprints or lakes, no problem. But polygons that form a continuous surface, such as landcover, administrative areas (e.g. counties, zip codes), soil types, or flood plains, big problem.

Why? Well, let’s take the example of a soil map. I downloaded this one from the Antrim County Community Center. It’s a 1928 Soil Survey of Antrim County, Michigan.

I zoomed in on an area of Antrim gravelly sandy loam, and drew my first polygon. Then I created a second polygon for the Lupton muck next to it.

Looks fine, except when we zoom in. The polygons don’t line up. Those empty spaces in between are called slivers. They are to be avoided at all costs. They will mess up feature counts and calculations of areas. They don’t represent the real world, and they look bad. However, they are virtually impossible to avoid if you are digitizing in this manner. You can try to by zooming really far in while you are drawing, and being really careful, but what a headache! Fortunately, the cost is not that high.

One way to avoid slivers is by the use of snapping. The snapping toolbar is available from the Editor dropdown menu in ArcMap. Here, I have turned on snapping to vertices in both the existing polygons and the new one I’m drawing (the sketch). Now, when I hover close to a vertex, the mouse will jump right on top of it. Snapping makes it impossible for you to create a vertex in your new polygon that is close to a vertex in any nearby polygon. It forces all vertices to be in the exact same place, or far away.

Note: This will make creating very tiny polygons difficult. If you run into trouble with this you will need to change your snapping tolerance by selecting Options in the Editor dropdown.

Snapping is enough to fix the problem if you have just a few polygons or a lot of straight lines (like rectangular buildings). But in the case of this curvy soil map, you’re still going to have to zoom in and be careful to “hit” every vertex. It only offers a small help.

A much better way: Don’t digitize the polygons. Digitize the lines between the polygons.

Create an empty line file, and don’t worry about attributes yet. Zoom in to the scale you plan to digitize at, and don’t deviate from that zoom. Turn snapping on so that intersections meet. Then, just pan around and draw.

Now

1. Create an empty polygon layer and add it to your map
2. Start editing and make the polygon layer your target
3. Select all the features in the line layer
4. Turn on the Topology menu, and use the Construct Features tool. This will create a polygon from every distinct closed area.

Note: Snapping is important for the tool to recognize these areas as closed. If your result has holes, it is because you missed a spot.

Congratulations! You have just about halved the number of vertices you had to draw, and created truly contiguous polygons. Now, add your attribute fields. It’s a good idea to save the line file in case you need to go back and make changes later.