Tutorial #A2 : orthophoto generation

Orthophoto Generation Workflow

 

Welcome to the 3DF Zephyr tutorial series.
In this guide, you will learn how to generate and customize an orthophoto with 3DF Zephyr.
This tutorial cannot be completed with 3DF Zephyr Free or Lite versions!

 

  • Introduction

3DF Zephyr can be leveraged to support topographic and land surveying measurements with drone UAV systems through orthophoto generation and adjusting image distortion and camera orientation. To explore this topic further, you can check out this quote from Wikipedia:
 


An orthophoto (or orthomosaic) is a geometrically corrected (“orthorectified”) picture whose scale is uniform. The photo has the same lack of distortion as a map, and, unlike standard images with a certain level of perspective, an orthophoto allows measuring true distances.

 

You need to have either a scaled and georeferenced point cloud or mesh in your project before generating an orthophoto in 3DF Zephyr.
 

We are going to use the same sample dataset we have leveraged in the Manage control points tutorial. Please, download and open the dataset below.
 

download datasetDownload Dataset – University of Udine Zip (77.3MB)
downloadDownload Dataset – University of Udine .Zep (487MB)
 
 

  • Step 1 – Start the Orthophoto generation Wizard

You will notice different orthophoto generation options from the “Workflow” (1) Menu:
 

  • Orthophoto from sparse points (requires generating sparse points cloud).

 

  • Orthophoto from dense points (requires generating dense points cloud).

 

  • True Orthophoto from Mesh (requires generating the mesh).

 


 

Once the “True Orthophoto from Mesh” (2) option has been selected, you will be allowed to choose the most suitable Orthophoto Generation Method from the “Orthophoto Wizard” window (3):
 

  • Use Axis: this option allows you to set the orthophoto plane perpendicular to the selected reference system axis.
  •  

  • Use Control points: you must place at least three control points on the surface on which you want to generate the orthophoto. All the GCPs in the project will be automatically loaded, and you can click the “Add”, “Remove” or the “Add all points” button according to the scenario you are dealing with.
  •  

  • Use Current view: the orthophoto plane will be set from the workspace camera in the 3D viewport.

 

Please, notice that this window is standard for all the above-mentioned methods. Once you have picked one of them, simply click the “Next” (4) button.
 
 

*TIP* How to Align your 3D Model to X or Y Axis?

In 3DF Zephyr, you can align your point cloud or mesh to the X or Y axis, which is handy for creating correctly aligned orthophoto from 3D models with a local reference system, e.g., building’s facades. After the point cloud or mesh creation and scaling, you can follow these steps:
 

  1. Create a distance by previously placing two control points as explained in the Manage control points and distances tutorial.

    NOTE: The two Control Points are supposed to be placed at the same height; therefore, the created Distance will be coherent with the facade surface. For example, you can use a window’s side as a reference.

  2.  

  3. In the Measurement tab, right-click on the created Distance and the “Align to” the desired axis (X or Y).
  4.  

  5. Once the model has been aligned to one of the reference axes, you can start the orthophoto wizard and enable the “Use axis” method to set the needed axis and proceed to create the Orthophoto.

 

Please, have a look at the Flow Tip video below as further reference:
 

 

 
 

  • Step 2 – Setting the Orthophoto

You will get a preview of the orthophoto once the Orthophoto Wizard (1) setting page appears, with a green rubber band (2) defining what will be included in the deliverable. Before doing anything, make sure to select the Reference Object and the Current Coordinate reference system you are interested in.

 

 

Please, check the “Export Orthophoto” (3) flag and then click one the “Generate Orthophoto” (4) button, you can choose both the file saving path and the image file format (.tif, .png, .jpg).

 

orthophoto_verruca_fortress
 

That’s all! If you want to customize your orthophotos or change the export options, next Step 3 – Learn to customize your Orthophoto will guide you through the main orthophoto settings.

 

 
 

  • Step 3 – Learn to customize your Orthophoto

The Orthophoto wizard settings are sorted out as follows:
 

 

    – The Position section allows defining the Width and Height size in pixels of the orthophoto according to the desired Ground Sample Distance, or you can set the GSD directly and let 3DF Zephyr calculate the parameter(s) for you. 
     
    Alternatively, you may set the orthophoto’s area, considering the green box top right corner and bottom left corner in the main rendering window, either by:

    • “Set the limits with rubber band” button: allows to dragging the green bands with the cursor, press “ESC” button if you incorrectly set the rubber band size. (Check the following Flow Tip Video for more details).
    • “Set the limits from control points” button: allows to choose two control points for delimiting the orthophoto area.

     

    The “closest up rotation axis” option can be used to rotate the orthophoto according to the chosen axis, or GCP. Tiles per height and Tiles per width define the numbers of vertical and horizontal tiles used to split the orthophoto into smaller tiles (fundamental for very big orthophoto or computers with a low amount of memory). Tiff pyramids level flag will allow storing inside the generated tiff orthophoto a pyramids levels layers, which can improve the display of raster data by retrieving only data at a specific resolution required for display.

     
     

    – The Export section allows choosing which files and formats will be generated as output:

    • Export point based Orthophoto: This flag should always be enabled if you need to generate the orthophoto; otherwise, it can be disabled,
      and only export the DSM or DTM.
    • Export Pdf
    • Export World file
    • Export Autocad Script File: By setting this flag, you can export a .scr file that will automatically load the orthophoto within Autocad, with the correct scale settings set in 3DF Zephyr. (For example, take a look at the dedicated Flow Tip Video).
    • Export .KML File: You can export the orthophoto in Google Earth by setting this flag. (For example, take a look at the dedicated Flow Tip Video).
    • Export Pdf
    • Export DSM File (Digital Surface Model): it’s possible specifying Elevation Axis for creating the Digital Surface Model.
    • Export DTM File (Digital Terrain Model): automatically generated according to the scene type. It’s possible to choose three different scenarios: Mountain area, Complex scene, and Flat terrain.
    • Export Single tile BigTiff will be available only for two or more tiles.

     
    Note: If the DSM/DTM files are generated, Zephyr will automatically open its DEM viewer utility when the process has finished.

     
     

    – The Advanced section gives you higher control over the orthophoto generation process. They are subdivided into:
     

    • Position parameters: In this subsection, it is possible to directly define the U0, V0, and U1, V1 coordinates and minimum and maximum depth (useful if you want to ignore “slices” of the reconstruction in your orthophoto).

    •  

    • Generation parameters: allows controlling every pixel’s color generation mode. This system is very similar to the texture generation system. We advise leaving the color balance with 1 camera per pixel; however, if you wish, you may use more cameras for a different color weight. If this happens, you may choose between an algorithm blending method:

    • Multiband: will use the mean to compute low frequencies and the best images to compute the high frequencies. This is usually a better choice than Mean because although it is less prone to lighting changes (usually just a little worse than mean), the resulting image will be more in focus and sharp by preserving high frequencies.

       

      Mean: will weigh the color source depending on the view angle and the distance. Although it’s robust to lighting changes, the mean won’t keep high frequencies, so will be less in focus and sharp.

       

    • The Select Cameras subsection allows you to select the cameras used in creating Orthophotos by tag as well.

    •  

    • The Export Options subsection controls the visibility of control points, grid and background color.

     
     

    When everything is ready, click “Generate Orthophoto” (3) and select the file destination folder in order to proceed.
    It is possible to save the Orthophoto in these image formats: jpg, png, tif.

     
     

    • Final notes

    The orthophoto generation procedure is relatively linear; however, we advise you to experiment in particular with the generation method to verify which one best meets your needs depending on the incoming dataset.
     
    In the Verruca Fortress case study; you can see an example of how the orthophotos created in 3DF Zephyr can be used in an applied scenario.