A layer might disappear from your map even though the data in it is fine. You could have a polygon layer with solid fills drawn on top of your road layer. The roads are there but you can’t see them. Maybe you drew a raster over a boundary layer and the boundaries disappear behind the raster cells. Layer order determines the order in which layers are drawn on the map and which features will be on top.
Most GIS software draws layers in the order in which they appear in the layer panel from bottom to top. In other words, the bottom layer is drawn first and then each layer drawn on top of it. You can think of the layer panel as being a stack of transparencies. Background layers such as basemaps, aerial photographs, and large polygons usually go toward the bottom, while features such as boundaries, roads, rivers, points, and labels tend to go toward the top. But, as always, the correct layer order depends on what you are trying to do with your map.
One rule of thumb is to place your polygon layer below line and point layers. Polygons usually cover more area than lines and points. It would make no sense to have a point covered up by a polygon or a line running underneath a polygon without being able to see it. However, there might be a case where a highlight polygon needs to appear above all other layers. In such a case, set the polygon’s fill to be transparent so you can still see the features beneath it.
Let’s try a quick test. Take a map with three layers: land cover, roads, and points representing cities. If the roads layer is below the land cover layer, what happens to the roads? Now, move the roads layer above the land cover layer. What happens to the roads? Then, try moving the city layer below the roads layer. What happens to the cities? Finally, move the cities layer back to the top. Use the same symbology for all three layers so you don’t accidentally affect the look of the map.
Another thing to consider is transparency and symbol style. Even if you place a polygon layer in the right position, it might block your raster layer if its fill is fully opaque. Change the polygon’s fill opacity to make it partially transparent, or use a light fill color, or just draw the outline of the polygon. Also, thick road symbols might overlap boundary layers. Big point symbols might overlap with one another and obscure important information such as a junction. Change the symbology once you have determined the appropriate layer order.
Finally, consider labels. You can arrange the layers in the perfect order, but if you have all three labels turned on, meaning road name, place name, and area name, they might overlap and make your map illegible. If you are using GIS software, you can adjust the label priority or overlap. For simple maps, however, turning off some labels is often a better solution. Don’t turn on all possible label fields. Think about what kind of information is important to the map reader and label those things.
When you decide on your layer order, think about the question you are trying to answer with your map. If you are creating a transportation map, it makes sense for routes and stop locations to be the most prominent elements. If you are mapping land cover, it makes sense for the color-coded polygons to be the main focus with roads used as subtle references. Once you have arranged the layers in the layer panel from bottom to top according to their visual prominence, review your map to see if you need to hide any layers. Go through the layer panel, and temporarily turn each layer off to see if it improves or detracts from your map.
