You'll find during this lesson that a rasterized tiled map service takes a lot of planning. Let's look at a few of the considerations needed to get a map ready for publishing as a tiled service. You'll download and examine a predesigned map and publish it as a service in preparation for making some tiles yourself.
The first question to settle is whether or not to make a tile cache at all. If the map is going to put strain on your server or take a noticeable amount of time to draw (these two often go together), then you need to consider making a tile cache. Most vector basemaps that give geographic context to your web map contain a lot of layers and fall into this category. This is one reason that splitting up your layers into basemap services and business layer services is a good idea; you can potentially cache the basemap while leaving the business layers uncached.
Is it necessary to cache the business layers, since that kind of data changes more frequently? Google used to do it with the Wikipedia layer in Google Maps [1]. With so many features (Wikipedia articles) to show, and with the amount of traffic Google Maps receives, it was burdensome on the servers to draw those points on the fly. (Sadly, the Wikipedia layer is no longer offered.)
In addition to high traffic scenarios, you can also consider caching business layers when the map covers a relatively small extent, the data doesn't change very often, or the data is displayed at small scales only. Layers like weather radar need to be updated frequently, but are rarely viewed at large scales and require relatively few tiles in the cache, thus the update can be performed in a reasonable amount of time.
There are a lot of decisions you need to make about how to set up your tile cache, but the first choice is the set of scales at which you are going to generate tiles. These scales represent the snapshots at which web users will see your map. They also determine how long it's going to take to create the cache, and which other web services the cache will be able to overlay. Ideally, you'll decide on your set of cache scales before you start designing your map.
Keep these things in mind when choosing a set of scales:
Creating detailed vector basemaps of the type that are typically cached presents a grand cartographic challenge. In contrast to paper cartography, in which the map has to be designed at just one scale, the web basemap has to be designed to look good at every scale in your tiling scheme.
Designing this type of multilevel basemap can require you to include varying symbols at different levels of your map. For example, a road might be represented with a 3-point line width at a large scale, a 1-point width at a medium scale, and may not be visible at all at a small scale. Since ArcMap does not allow scale-dependent symbols, you'll sometimes need to add multiple copies of the same layer into your map, set different scale ranges on them, then assign appropriate symbols for each scale range.
It's also important to choose muted colors for the base map that look good, but do not overwhelm other layers placed on top. Go to Google Maps: Designing the Modern Atlas [2] to see some examples of how the Google Map design has toned itself down over time to be more accommodating to overlays.The Esri Light Gray Canvas basemap is another study of designing a basemap specifically as a backdrop for more important thematic or operational layers.
When web mapping exploded during the past two decades, some cartographers expressed their chagrin at the simple, uniform maps churned out by websites. Some may have thought their very jobs and livelihood were threatened. However, the years have shown that cartography holds a critical place in web mapping. Projects like the OpenStreetMap terrain layer [3] and the Esri World Topographic Map [4] incorporate very advanced cartographic techniques. In a sense, map tiling gave cartographers a ticket to ride in the web world, since these detailed maps would be too slow to serve dynamically.
No wonder some GIS professionals shrink at the thought of trying to design such a map on their own. Some organizations that lack an in-house cartographer have just limped along with the same symbols they used when more primitive map server technology was available. Others have imitated the colors and symbols of the ubiquitous Google Maps in their own basemaps (perhaps in response to a manager's demand, "Make our maps look like that!").
In response to queries about how the ArcGIS Online basemaps were constructed, Esri has released sample ArcMap documents using all the ArcGIS Online base map symbols. People can insert their own data into the map or simply copy the symbol settings into their own maps. Examining one of these maps provides a good lesson in multilayer basemap design.
In this part of the lesson, you'll download and examine a map template that Esri has provided for the ArcGIS Online street map. This sample map covers the Little Rock, Arkansas region. You'll then publish the map as a service and get it ready for creating tiles in the next section of the lesson.
Links
[1] http://maps.google.com
[2] http://www.core77.com/blog/case_study/google_maps_designing_the_modern_atlas_21486.asp
[3] http://mike.teczno.com/notes/osm-us-terrain-layer.html
[4] http://www.arcgis.com/home/item.html?id=6e850093c837475e8c23d905ac43b7d0
[5] http://www.arcgis.com/home/webmap/viewer.html