Many archaeological projects now use photogrammetric modeling to record stratigraphic units and other features during the course of excavation. In another post we discussed bringing photogrammetric or laserscanning derived models into a GIS in situations where you don’t have precise georeferencing information for the model. In this post we will demonstrate how to use bring a photogrammetric model for which georeferenced coordinates are available, using Agisoft’s Photoscan Pro and ArcGIS.

[wptabs mode=”vertical”] [wptabtitle] Load Photos[/wptabtitle] [wptabcontent]Begin by adding the photos used to create the model to an empty project in Photoscan.

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[wptabtitle] Align Photos[/wptabtitle] [wptabcontent]Following the Photoscan Workflow, next align the images. From the menu at the top choose ‘Workflow’>’Align Images’. A popup box will appear where you can input the alignment parameters. We recommend selecting ‘High’ for the accuracy and ‘Generic’ for the pair pre-selection for most convergent photogrammetry projects.

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[wptabtitle] A choice[/wptabtitle] [wptabcontent]At this point there are two approaches to adding the georeferenced points to the project. You can place the points directly on each image and then perform the bundle adjustment, or you can build geometry and then place the points on the 3d model, which will automatically place points on each image, after which you can adjust their positions. We normally follow the second approach, especially for projects where there are a large number of photos.
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[wptabtitle]Build Geometry[/wptabtitle]
[wptabcontent]Under ‘Workflow’ in the main menu, select ‘Build Geometry’. At this point we don’t need to build an uber-high resolution model, because this version of the model is just going to be used to place the markers for the georeferenced points. A higher resolution model can be built later in the process if desired. Therefore either ‘Low’ or ‘Medium’ are good choices for the model resolution, and all other parameters may be left as the defaults. Here we have selected ‘Medium’ as the resolution.

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[wptabtitle]Get the georeferenced points[/wptabtitle]
[wptabcontent]When the photos for this model were taken, targets were places around the feature (highly technical coca-cola bottle caps!) and surveyed using a total station. These surveyed targets are used to georeference the entire model. In this project all surveyed and georeferenced points are stored in an ArcGIS geodatabase. The points for this model are selected using a definition query and exported from ArcGIS.

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[wptabtitle]Add the georeferenced points[/wptabtitle]
[wptabcontent]On the left you have two tabbed menus, ‘Workspace’ and ‘Ground Control’. Switch to the the ‘Ground Control’ menu. Using the ‘Place Markers’ tool from the top menu, place a point on each surveyed target. Enter the corresponding coordinates from the surveyed points through the ‘Ground Control’ menu. Be careful to check that the northing, easting and height fields map correctly when importing points into Photoscan, as they may be in a different order than in ArcGIS.

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[wptabtitle]Local coordinates and projections[/wptabtitle]
[wptabcontent] In practice we have found that many 3d modelling programs don’t like it if the model is too far from the world’s origin. This means that while Photoscan provides the tools for you to store your model in a real world coordinate system, and this works nicely for producing models as DEMs, you will need to use a local coordinate system if you want to produce models as .obj, .dae, .x3d or other modeling formats and work with them in editing programs like Rapidform or Meshlab. If your surveyed coordinates involve large numbers e.g. UTM coordinates, we suggest creating a local grid by splicing the coordinates so they only have 3-4 pre decimal digits. [/wptabcontent]

[wptabtitle]Bundle Adjust – Another Choice[/wptabtitle]
[wptabcontent]After all the points have been placed select all of them (checks on). If you believe the accuracy of the model is at least three time greater than the accuracy of the ground control survey you may select ‘update’ and the model will be block shifted to the ground control coordinates. If you believe the accuracy of the ground control survey is near to or greater than the accuracy of the model, you should include these points in your bundle adjustment to increase the overall accuracy of the model. To do this select ‘optimize’ from the ‘Ground Control’ menu after you have added the points. After the process runs, you can check the errors on each point. They should be less than 20 pixels. If the errors are high, you can attempt to improve the solution by turning off the surveyed points with the highest error, removing poorly referenced photos from the project, or adjusting the location of the surveyed points in individual images. After adjustments are made select ‘update’ and then ‘optimize’ again to reprocess the model.

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[wptabtitle] Continue to…[/wptabtitle]

Continue to PhotoScan – Building Geometry & Texture for Photogrammetry

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[wptabtitle]Import into ArcGIS[/wptabtitle]
[wptabcontent] From ArcToolbox select ‘3D tools’ and ‘From File’ and ‘Import 3D Files’. Select your model and set the output as a new multipatch feature class in your geodatabase.

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[wptabtitle]Moving the model[/wptabtitle]
[wptabcontent]Once the model has successfully imported into ArcGIS, you can move it to the correct real world coordinates by selecting ‘Edit’ and ‘Move’ and adding the appropriate x and y values (those removed before to make the modeling software happy). If the difference between the two coordinate systems is large, ArcGIS produces an error stating that the new coordinates are outside the spatial domain of the feature class. In this case, create a new feature class (with your real world projection) and copy the model into it. You should now be able to move the model to the correct coordinates.

This is also an opportune moment to put all your models, which you’ve been importing into individual feature classes, into a single feature class.[/wptabcontent]

[wptabtitle]Viewing the models[/wptabtitle]
[wptabcontent] You can now view the models in ArcScene and manage the data as you would any other feature class in ArcGIS.

Hint: If the model appears bizzarely blocky or is empty after you add it to the scene or map, and you are sure you did everything correctly, there is probably a conflict between the scene or map spatial data frame and the objects spatial reference. Open a new scene or map, or change the spatial data frame to resolve the problem.
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[wptabtitle]Logs and Metadata[/wptabtitle]
[wptabcontent] Photoscan logs all its activities in the console (at the bottom of the screen). You should save this information to a .txt file for use as metadata. Similarly, you should store the .exif data for the photos and the calculated camera positions, which may be exported as .xml

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• Open ArcScene
• Add your polygon data from shapefile, geodatabase, or SDE connection
• “Right Click” on the layer in the Table of Contents ->
  “Select” = ‘Properties…’ (very last option)

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[wptabtitle] Extruding Features[/wptabtitle] [wptabcontent]

• “Select” ‘Extrusion’ tab on the Layer Properties
  Menu
• Check the box and then either extrude feature by a
  set value or pre-determined expression. (You can also select and/or construct attribute expressions using the ‘Expression Builder’ button)
• Visually inspect extruded features in the Dataframe
  and ensure they look to be correct.

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[wptabtitle] Convert to feature class[/wptabtitle] [wptabcontent]

• In ArcToolbox > 3D Analyst Tools > Conversion >
  “Layer 3D to Feature Class” is the tool we will use to
  convert the extruded features to a ‘Multipatch’ feature class.
• Input the corresponding Feature layer to be converted into a Multipatch, put it in a familiar directory for output.

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[wptabtitle] Convert to Collada[/wptabtitle] [wptabcontent]Again in the ArcToolbox > Conversion > To Collada > ‘Multipatch to Collada’ tool is what we will use to output a file in a format Google Sketchup® will recognize.


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[wptabtitle] Import to Sketchup[/wptabtitle] [wptabcontent]

• Open Google Sketchup® and choose a template that is in the same units as your projection in ArcScene (i.e. meters, Feet and Inches, etc.)
• In Sketchup > File > Import…
• Navigate to where you just exported the Collada file from ArcScene and make sure your file type is COLLADA in the drop down menu
• “Select” > Options…
• Here you will want to decide if you want to merge coplanar faces – typically for texturing the outsides of builds you do want to merge

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[wptabtitle] Rendering tasks[/wptabtitle] [wptabcontent]Perform all necessary rendering tasks. Be sure to group all elements before saving.[/wptabcontent]
[wptabtitle] Returning to ArcScene[/wptabtitle] [wptabcontent]Getting the model back into ArcScene is almost the exact opposite first we want to export our finished model in Sketchup by “Selecting” File > Export > 3D Model

• Name the File something relevant and save it in a familiar directory as a COLLADA .dae file
• Add an empty or populated multipatch feature class into ArcScene or ArcGlobe.
• Begin a 3D edit session by clicking the 3D Editor drop-down and click “Start Editing”.
• Click the Edit Placement tool on the 3D Editor toolbar.
• Click the multipatch feature in the Create Features window.
• The Insert tool will appear in the Construction Tools window.
• Select the Insert tool under Construction Tools.
• Click the desired location of the multipatch model in the 3D view.
• When prompted, navigate to the location of the supported 3D model file on disk.
• Select the model and click Open.

***Note you will add the same model over and over again with ever click until you deselect the “Insert Tool”[/wptabcontent]
[wptabtitle] The Result[/wptabtitle] [wptabcontent]By following these steps you can bring your model back and forth between the two programs.

Original model in ArcGIS

Post-Sketchup Model

The NITF (National Imagery Transmission Format Standard) format is commonly used for georeferenced imagery and has the ability to store a large amount of ancillary data and metadata along with the image itself, all in one file. One of the more useful pieces of ancillary data optionally stored within an NITF file is known as RPCs (Rational Polynomial Coefficients). With the use of RPCs and elevation data for the area, complete orthorectification is possible. For example data, see CAST’s “Corona Atlas of the Middle East” and choose from the large number of NITF files available for download.

Viewing and creating NITF orthoimage in ArcGIS

Written for ArcGIS 10

1. Open ArcMap and add one or more NITF image files.

2. Add a DEM covering the area.

3. Right-click the image name in the Table of Contents and choose Properties.

4. Under the Display tab in the Layer Properties dialog, make the following three changes in the “Orthorectification” box and click OK:

1. Check “Orthorectification using elevation”.

2. Click radio button for “DEM” and use the drop-down menu to choose the appropriate DEM.

3. Make sure “Geoid” is unchecked.

These Layer Properties will need to be set for each NITF file you add to ArcMap.

5. Each NITF image with the settings from step 4 above should now appear orthorectified. To make this permanent, they will need to be exported to create a new file and format (ArcMap will not export to NITF). To do this, there are two options:

1. Use the “Export Data” dialog found by right-clicking the NITF file’s name in the Table of Contents, holding the mouse over “Data” in the context menu, and clicking “Export Data”. Options for extent, spatial reference, cell size, file location, name and format are available in this dialog. Choose the desired settings and export.

2. Use the “Create Orthorecitified Dataset” tool in the ArcToolbox. See the ArcGIS help for help with this tool.

As another option, see the free “NITF for ArcGIS” extension for Desktop and Server available for download here: http://www.esri.com/industries/defense/nitf