take the edge off Survey, GIS

With aerial photos or scanned maps, it is common to have a large number of tiled images. Using FDO’s to bring the images is in a great way to improve speed but when we are talking about many images, it can still have issues. One of these is the fact that even though FDO’s are a serious improvement over the old fashioned insert image function, it will still slow the AutoCAD Civil 3D session down, particularly for panning/zooming - another is that every time the DWG file is opened, all the images have to be resolved regardless of whether they are displayed or not - for a large area (a State in Australia for example), this can take a long long time. I know people who go for a coffee while the system is sorting itself out.

In this example, each square on the grid represents a 50MB image. We don’t want all this images on the screen at the same time but we do want to be able to zoom into an area and have them appear for just this region without too much fuss.

This solution is not simple - we use Mapguide to do the work externally to the Civil 3D session. It is well worth the effort though - the improvement in speed is amazing.

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the problem in more detail…

A company who uses Civil 3D is trying to examine water services over extended areas. The Mapinfo people who sit next to them have done some clever hacks to enable their software to only query in a relevant image when they are zoomed in to a reasonable scale - this works very well and, more importantly, very efficiently in terms of speed.

With Map/Civil 3D, there are a few things we can do to improve performance like limiting the scales the images show up at but the process is still inefficient. The main problem is that every time the drawing file is opened, all the FDO layers are queried, even if they’re turned off. As the AutoDesk software has progressed so far in terms of general GIS, we expect it to be able to stream images like Google Earth…

the solution in conceptual terms

The standard scenario is to store the images on a server somewhere and leave Civil 3D all the processing work. In essence, we are introducing a mapguide server to the system to take the edge off the workload in image processing for Civil 3D. We form the link to Civil 3D by using WMS.

In the diagram above, Civil 3D is doing all the work - the File server is literally just holding the files.
In the second diagram, the File server is also a Mapguide Server so it is capable of doing some processing work. This means that the Civil 3D workstation does not have to waste resources on image processing - i.e. the whole thing runs a lot smoother.

how much general mapguide do you need to know for this?

Not much - the standard Mapguide project works off 4 steps.

• Data…(tell the software where the data is)
• Layers…(apply stylisation)
• Maps…(compile a number of layers into a map collection)
• layouts…(publish to a standard web browser)

To achieve what we need, we only need to look at the first 2, Data and Layers. If you use Mapguide Enterprise, you can drag and drop the images to create an upload procedure which creates the Data Sources and Layers for you.
This method physically copies the files to the mapguide library folder (something like C:\Program Files\ Autodesk\ MapGuideEnterprise 2009\ Server\Repositories\ Library\DataFiles).
On my testing, this method was significantly faster than doing a data connection to the original location. The open source version of Mapguide can also process the images but the raster FDO provider is not as funky.

WMS…

Web Map Service…"a portrayal of geographic information as a digital image file suitable for display on a computer screen. A map is not the data itself….”
The Map produced by the WMS is an image so if our data was a shapefile, we’d still be getting a raster representation. What we’re doing here is taking an image of a series of images.

The steps for setting it up are covered very well in the Geospatial Made in France Blog. (There is a translate to english button on the top left).
In essence, it’s a matter of enabling the WMS in the mapadmin section and then going to the properties of the various Layers to enable the ‘publish via Web Mapping Service’ tickbox and enter some settings. There is one tricky part though…

The Tricky Part…

The tricky part is the co-ordinate systems. First of all, the images cannot be transformed to a different system by mapguide. This is no problem - if push comes to shove, we can use Raster Design to get them (and the correlation files) into the correct system before uploading to mapguide.
The confusing part is that the WMS service needs the co-ordinate system in ESPG.

<Bounds SRS="EPSG:4326" west="XXX" south="XXX" east="XXX" north="XXX"/>

In the Layer properties, there is a section to type in the Bounds. The East, West, North and South values are just the extents of the space occupied by the images. The “SRS="ESPG:4236″ part refers to LL84. Working out how to change this to a different system is not obvious. The good news is that Gwenael has covered this in his Blog in a separate post here.

The finished result…

The drawing opens up quickly and the images stream quickly. In this case, I have set the scale range so the OS maps only show up when I’ve zoomed in to an appropriate scale and it works perfectly.

a good way to get started…

The sample Mapguide data set for Sheboygan has got WMS enabled for most of the layers - try loading these into a Civil 3D session first to get a feel for how it works. These samples are vector layers but the process will be the same.

I’ve only tested this on my home PC. I’ll be doing a rollout soon on a real network for a major company so we’ll calibrate the improvement in speed and post it up here.

cheers,
SeanT