Thematic layers (also known as business or operational layers) go on top of the basemap.
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Keeping these two tilesets separate takes less disk space and makes it easier to update the imagery. (In Google Maps, this appears when you check the Labels item). For example, you may have a tiled layer with aerial imagery and a second tiled layer with a vector road overlay that has been designed to go on top of the imagery. Sometimes two tiled layers will work together to form a basemap. You can see a vector basemap in action if you look at the Google Maps app on a smartphone. In the past couple of years, it has become fashionable in some applications for the server to send the basemap as chunks of vector coordinates, sometimes known as "vector tiles." Displaying the basemap as vectors instead of a rasterized map allows for more flexibility in visualization, such as being able to rotate the map while the labels stay right-side-up. Lesson 5 will explain tiled maps in greater depth and give you a chance to make your own. These tiled maps consist of often thousands or millions of predrawn images that are saved on the server and passed out to web browsers as people pan around the map. The most common basemaps you've used online are vector road maps and remotely sensed imagery.Īlthough a basemap may consist of many sublayers (such as roads, lakes, buildings, and so forth), these are often fused together into a rasterized set of tiled images and treated as a single layer in your web map. In other words, it is usually not the main reason people look at your map, but your map would be difficult to interpret without it. BasemapsĪ basemap provides geographic context for your map. Let's take a closer look at these three things-basemaps, thematic layers, and interactive elements-to understand how each is created and displayed. You might additionally decide to include a set of interactive elements such as popups, charts, analysis tools, and so forth. In contrast, thematic layers (the layers that are the focus of the map) are brought in as one or more separate web services and placed on top of the basemap. Layers whose sole purpose are to provide geographic context are grouped together and brought into the map as a single tiled basemap. To address this problem, most web maps break apart the layers into groups that are handled very differently.
The server requires precious time to iterate through all those layers, retrieve the data, draw it, and send the image back to the client. Your powerful desktop machine may be able to handle the drawing of this kind of map however, performance will be unacceptably slow if you try to move the map directly to the web. Many people who have worked with desktop GIS packages are accustomed to adding dozens (or even hundreds) of layers to the map and switching them on and off as needed.
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For example, if you've only used ArcMap or QGIS in the past, you may not be accustomed to thinking about broadband speeds or sharing the machine with other people.īy far, the biggest challenge for new web mappers is understanding the amount of data that is displayed in their maps and how to get all that information drawn on the screen of a web user in sub-second speeds. These considerations can sometimes take people by surprise. In a web map, you are limited to the display technologies supported by the client application, which may be a basic web browser.In a web map, performance may be affected by other people using the server at the same time.Your map performance is therefore limited by the availability and speed of all the servers you're using. In a web map, you may be pulling in information from several different servers.In a web map, any information you see has to be sent “across the wire” from the server to your browser, introducing latency.Building and using web maps is different from working with a desktop GIS for a variety of reasons:
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