三维建模之SketchUp™ and the Multipatch

SketchUp™ and the Multipatch

Putting this newsletter together has been very exciting for us. Thanks to the efforts of @Last Software and their product called SketchUp™, multipatch buildings are about to become a reality. In the very first 3D GIS Newsletter, we ended with the question, “What is a Multipatch?” We followed up with a brief answer in the second issue, but going much further at that time was not appropriate. The situation changed dramatically and we want to be the first to share this new information with you.

First of all, the multipatch is a type of shapefile that dates back to the original shapefile specifications. For those of you who want to know more, give your browser the following address: http://www.esri.com/library/whitepapers/pdfs/shapefile.pdf and scroll to page 20. If trying to understand triangle strips, triangle fans, etc. is not your thing, all you really need to know is that these are specifications for how to “fold” an object, drawn on a flat surface, to achieve a 3D feature. This is the information that the programmers at @Last Software need to write the code that allows you to save your SketchUp work in a multipatch format.

For the past few months, the folks at @Last Software have been working on an ArcGIS interface for SketchUp™. What began as improvements in the process of importing SketchUp™ output to ArcGIS 3D symbols has grown significantly. Now the goal is the creation of a plug-in to SketchUp and a toolbar for ArcGIS that allows native SketchUp™ files to be added as either a 3D symbol or a multipatch.

Using SketchUp™ is easy

Following the 3D demonstration at one of the rollout seminars for ArcGIS™ 9.0, one person remarked that he thought it might take a year to create the Honolulu example he had just seen. This person had obviously not seen SketchUp™ in action. With just a few pieces of dimensional information, a completed multipatch (or symbol) could be just a short time away.

Viewing the Style Manager in "Details" mode allows for the display of category names.

SketchUp also has a great set of training videos that come with the software. These videos make getting started a very easy process.

Distinguishing a 3D symbol from a multipatch

We could expend the space in this newsletter to show you a building as a symbol and the same building as a multipatch, but there wouldn’t be anything different to see. They look the same. The difference is in how they are used in ArcScene or ArcGlobe. Both the multipatch and 3D symbol versions of a building know their size and dimensions. The multipatch building, since it is a feature in a feature class, knows it’s X, Y and Z coordinates in real world space. The 3D symbol does not know this information. It is only used to represent a point in a point layer or is simply placed on the landscape as a graphic.

Perhaps more important is the fact that the multipatch can have it’s own attributes and thus it can participate in geoprocessing operations. For those of you that attended one of the rollout seminars for ArcGIS 9.0, the Honolulu buildings were multipatch features and thus they could participate in the radio signal propagation model that evaluated a proposed broadcasting antenna. Had the buildings been symbols, the signal evaluation would not have been able to consider the impact of the buildings.

Initially we found the concept of a multipatch difficult to understand. Perhaps the most important thing to realize is that a record in a multipatch feature class is really no different then any other piece of GIS data. A traditional point, line or polygon in ArcMap is essentially the same thing as a multipatch building viewed in 3D: they are nothing more then a record in a feature class with certain attributes attached to them.

How do you make the multipatch spatially aware?

The folks from @Last Software have been very clever in this process. The first step is to create a new geodatabase and multipatch feature class in ArcCatalog™ that can receive the building models. As with all feature classes, the map projection is defined. To make the building spatially aware (know it’s X, Y and Z), the user starts by selecting features from layers, in ArcMap™, ArcScene™ or ArcGlobe™ and then importing those selected features into SketchUp™. Imported layers can include a tin. If one of the layers includes a building footprint, it can be used right away to start constructing the building model. If a footprint is not available, the footprint is drawn using another layer (or layers) as a reference. Once the building is completed, it is selected from the background layers and saved as a multipatch. When the building is added to the 3D environment of ArcScene™ or ArcGlobe™, it will retain is spatial understanding inherited from the other layers and be properly positioned on the landscape.

Updating a multipatch

If it becomes necessary to change or modify the multipatch building in a scene, all one has to do is to select the feature (building) and bring it back into SketchUp™ for modification. Once the changes have been made, the building is placed back on the landscape, replacing the previous version. This functionality makes SketchUp™ the editing tool for the multipatch.

SketchUp's market penetration

One of the very exciting things about SketchUp™ and its use in GIS is the fact that it has already sold very well in its original target market, the architectural community. At the American Institute of Architecture convention, held in Chicago this past June, they sold in excess of 250 copies from the trade show floor and their total worldwide sales numbers are already well into five figures. We can’t help but think of each of these existing SketchUp™ users as people that can provide content for our 3D GIS. Chances are, new and proposed buildings in your area may already exist in SketchUp™ format, ready to be converted to multipatch and added to your 3D environment.

We sell and support SketchUp™

SketchUp is a very affordable product. At $495, list price, coming on a CD with training videos, it fits in almost every budget. The plug-in to enable GIS and multipatch integration will be available in the next few weeks, and it is free! We are now @Last Software’s only domestic reseller. We were added to concentrate on the GIS market. If you want to try the software, just e-mail us and we will send you a CD with the software for you to try for 8 hours of actual use. The CD will also hold the training videos to help you get started.

If you are going to the ESRI User’s Conference in San Diego, make sure you stop by the @Last Software booth to see SketchUp™ in action. Tell them that the folks at Green Mountain GeoGraphics recommended you stop by.

For those that are interested in a real-world example, a case study of some local 3D GIS work that we have done is featured in the Summer 2004 issue of ArcNews. To view this article online, go to http://www.esri.com/news/arcnews/summer04articles/essex-vermont.html