Technical Tips

Introduction

Base Heights

Limitations in the Current release of 3D Analyst

Introduction

The visualisation application that supports 3D Analyst (ArcScene) is a welcome addition to the ESRI family of geospatial software products. This extension represents far more than a 3D rendering software suite, many of which have been available for decades, but the migration of the richness of a GIS (identification, query, analysis and reporting) into a 'realistic' medium for visualisation.

Asset managers and natural resource managers can take a step into virtual reality with this new tool. The 'wow' reaction of clients from demonstrations of 3D visualisations confirms that this technology opens a whole new way for natural resource and asset managers to view and interact with their data.

The tips in this addition focus on two areas:

Establishing a realistic surface (setting base heights) and,
Some of the limitations of the current release of ArcScene.

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Base Heights

The basis of any 3D visualisation in ArcScene is concept of draping a dataset over a 3D model of some terrain surface. The 3rd dimension (or Z value in a X, Y, Z coordinate system) is modelled as heights above some base value (usually sea level). The dataset that defines a 3D model is the base height source for the visualisation (in the case of terrain this is the Z or elevation value). There are two ways that this information can be sourced. The first is to use a Triangulated Irregular Network (TIN) that has been generated at some point. The other source can be a raster dataset such as a Digital Elevation Model (DEM) that provides the data surface. If you have contour data you can create either a TIN or DEM directly using functions included with 3D Analyst.

For high resolution 3D models it is best to use a TIN, as the surfaces rendered on this foundation will have a more realistic look. For low resolution models, the raster option is best as this will generally provide a faster response and will not overly tax your computing resources.

For most applications you will have only one source for your base heights, that is, a digital model of the surface terrain.

Something to remember is that Australia's terrain is generally quite flat! You will need to apply some type of vertical exaggeration to make 3D visualisation worthwhile in most cases other than highly localised projects. The easiest way to exaggerate the layers you drape over your surface is to use the Z Unit Conversion control in the Properties/Base Heights Tab (see Figure 1). Set the first option to custom and add some number to the second. Experiment to get the best effect. I have found that numbers between 2 and 4 produce 'realistic' looking results. Remember to apply this factor to all datasets you wish to use in the 3D environment.

Figure 1: Setting properties such as base heights, vertical exaggeration and floating offset

I would strongly recommend that once you have assigned the appropriate properties for each dataset, that you then create Layer files for each dataset or group of datasets. If you wish to use these data again, the layer files will save you time as they record the settings and symbology for individual layers or layer groups. We will further explore how to generate layer files in a future Technical Tips note. The example scene below is Mt Arapiles within Tooan State Park in Victoria.

Figure 2: TIN of Mt Arapiles (100 Metre Contours)

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Limitations in the Current Release of 3D Analyst

Legends: The current version of ArcScene has no capability to creating a legend. This is an obvious limitation, particularly if you have spent a great deal of time symbolising a set of spatial datasets in ArcScene. I have contacted ESRI technical support regarding this and they have indicated that this area of ArcScene is currently under development.

File Sizes: ArcScene does not support files greater than 300mb so large areas of high resolution or very high resolution imagery are not always possible to view as drapes.

Node Limitation: A maximum of 300,000 nodes are supported. This can have obvious implications, for example, for large extents of complex data such as road networks, hydrology networks or property parcels. Any large dataset will cause you significant performance issues.

Export Options: The current version does not to support the full range of 3D export options. ESRI have confirmed that these functions will be available at the next release of ArcScene.

Computing Resources: 3D Visualisation is a resource intensive exercise. The more RAM the better. RAM of 500Mb plus on a Pentium 3 or better provides adequate performance.

The above limitations don't detract from the effectiveness of this 3D visualisation tool and its ease of use. This level of interactive 3D visualisation historically has required very expensive software and high-end computing equipment. ArcScene's ease of use and affordability confirms that this powerful capability is now available to a much larger group.

The potential applications for 3D Analyst have merely been touched upon in this article. Three potential areas of application development are:

Natural Resource and Land Management: managers can visualise an area of interest and quickly develop an understanding of the area's terrain. ArcScene also provides a suite of tools to undertake what otherwise would be very complex tasks. Seen-area analysis, line-of-sight profiles, surface area calculations and volume estimation are just a few of the possible applications.

Property Development: developers, architects, engineers and urban designers are able to visualise development plans in three dimensions including evaluating options for siting buildings, determining landscape views and modelling the visual impacts of new projects. Rapid production of multiple 3D perspectives of prospective developments are now possible Fly-throughs of various scenarios can be developed and used to create promotional movies and other information products.

Sub-Surface and Marine Management: analysts can build fully developed, intelligent, 3D models of an off-shore drilling sites, mines and other subsurface structures. The analysis of vertically stratified marine environments, groundwater aquifers and subsurface drainage networks are clear beneficiaries of this technology.

In the future additions of the SV Newsletter, we will focus on other aspects of this improved visualisation application. Topic areas being considered include seen-area analysis, rendering, symbolising your surfaces to best effect and mapping in 3D.

For further information about ArcScene, a demonstration or generation of a base TIN, please contact Milos Pelikan.

Figure 3 : Hill Shaded View of Mt Arapiles

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Copyright © Spatial Vision, Tuesday, 24-September-2002