Positioning in a global context

Most often, Revit models have been correctly configured with the appropriate local grid coordinates X (eastings) and Y (northings) directions. However, to correctly position a Revit model in a global context, the iTwin connector requires a global position, which cannot be obtained solely from the X and Y values.

Without this information, the models will therefore be positioned at the default location set in Revit.

Image 1 - Revit model at default set location

To achieve a correct positioning, the model file must be updated with the correct latitude and longitude values. Unlike other design applications where a user can select a global coordinate system from a built-in library, Revit relies on the input of latitude and longitude values. In return, these settings can be used not only to create a correctly positioned iModel but also for Revit solar studies and analysis. Furthermore, when exporting to IFC files, global coordinates will be made available.

Image 2 - Correctly positioned Revit model

Revit Coordinates

To achieve a correctly geolocated Model, typically a model file needs:

• Shared coordinates established
• Survey Point set to local coordinates (X and Y location)
• Latitude and Longitude set

Survey Point (SP)

To set specific coordinates for the SP:

• Select the SP
• Unclip the SP

Image 3 - Unclipped Survey Point

• Use the Specify Coordinates at Point tool to set the coordinates for the SP. This is usually a known point supplied by a qualified surveyor indicating the nearest survey mark.

Image 4 - Specify coordinates at point tool

• Set the North/South (Y) and East/West (X) orientation, the elevation and finally the angle to true north

Image 5 -  - Specify coordinates

This will update the SP with the coordinates specified.

• Clip the SP again to ensure it cannot be accidentally modified

Image 6 - updated Survey Point with example UTM84 coordinates

Project Base Point (PBP)

The PBP is often used as a project coordinate point indicating key features on-site or a building set-out point. Measurements can then be taken from that point (PBP).

Image 7 - Project Base Point

The PBP will always be relative to the SP. Moving the PBP while the SP is in its clipped state will have no effect on the SP itself. The PBP only plays a minor role when creating iModels.

Verify the location in a CAD application

• Export a site drawing to a DWG file with shared coordinates enabled

 

Image 8 - Export settings - Units & Coordinates

• Attach the exported file as a reference in MicroStation or Civil3D (AutoCAD)
• Select and enable a suitable global coordinate system from the library
• Turn on the background map

Image 9 - DWG export with background map overlay

 

A MicroStation or C3D file with the correct GCS can be used to acquire the latitude and longitude relative to the SP.

<<Insert link to training session>>

Setting latitude and longitude

The latitude and longitude must be set before the Revit model can be connected to an iModel.

Image 9 -  Updated lat / long in Revit 

This will shift the location from the Blue Hill Cemetry near Boston to Coffs Harbour, NSW.

The location settings can be found and modified in Manage-> Location.

• Define the location by selecting Default City List and entering the lat / long coordinates
• Click OK and open the menu again to verify the settings
• Check that the pin is pointing to the correct location

 Image 10 - Location and Site settings

Resulting iModel

The finished iModel will be positioned at the defined location, with the SP matching the latitude and longitude values.

Image 11 - Coordinates measured in an iTwin app

Eyeballing Method

Additionally, this method can be used in cases where there are no local grid (X and Y) coordinates available. The SP can remain at 0,0 (the default location) BUT the latitude and longitude must be set.

Here, the lat / long can be grabbed from Google Maps, using an approximate location where the model can be positioned.

• In Google Maps click at the point, where the SP should be located
• Right-click and copy the coordinate values
• Add the latitude and longitude values to the Location and Site menu

Image 12 - Grabbing coordinates from Google Maps

Please note that this approach is not accurate and should only be used in cases where it is impossible to acquire accurate location information. Additionally, please be aware that measuring coordinates in the iModel will not be possible.

Notes

Consider using the SP for everything related to a 'global' positioning of a project. The SP can be set to the location of a known survey marker or an existing fixed infrastructure feature, such as a utility access hole cover. Something with a known location, easily verifiable on site.

Use the PBP for everything 'project' related, such as linking Revit files with each other or to continue 'working in CAD'.

Independently verify any survey information.

There are multiple Revit workflows available describing the setup and use of shared coordinate systems and location settings. To discuss all would go beyond the scope of this document, and further advice can be found on the Autodesk website.