Using April Tags as Control Points


Introduction

In this example, the user visits a site every week and leave Targets to automate the survey points input. They are unable to use QR codes because the acquisition settings will not allow them to have the required resolution (150 pixels) to be detected, so they use AprilTags instead (35 pixels).

The problem with this is that unlike QR Codes, AprilTags cannot carry information, such as coordinates, within the code itself which means that coordinates must be added afterwards.

Editing the Survey points created by AprilTags detection can be very time consuming, but this workflow will show you how to automatically merge the survey points extracted from April Tags with Control points previously added.

Instructions

  1. Create a text file containing a list of the control points, named with the format “AprilTag #X” and separated by commas (see the example below). Targets must match the name of the target on site.

NOTE: If TXT File comes from the export of a Target extraction in iTwin Capture Modeler, you will need to edit the file to match the format mentioned.

A number with numbers on it

  1. In iTwin Capture Modeler, once you have added the photos to a block, go to Survey tab and click on the “Import surveys from file” button. Then, Select “Custom Text Format (Wizard)”.

A screenshot of a surveyAI-generated content may be incorrect.

  1. Follow the instruction in the dialog to set up the format, making sure to select comma as separator. Remove any header lines, select the correct Coordinate System and assign the correct role to each column.

 A screenshot of a computerAI-generated content may be incorrect.

  1. Once you have added the control points data, perform an aerotriangulation selecting:
    • Positioning/Georeferencing: Control points (including from targets) on Adjustment constraints and Check the box “Use targets (QR Codes, AprilTags or ChiliTags)”.
    • Settings: Make sure Targets extraction is set to detect your Targets.

Screens screenshot of a computerAI-generated content may be incorrect.

  1. The result should be a merge of the detected targets from the AT with the survey data you added through the text file. If some targets are not found on the Photos, such as Target #1 on this example, it will not have any photos related to it, and can be removed. A screenshot of a computerAI-generated content may be incorrect.
  2. Now you have your data adjusted to the control points using April or Chili Tags without having to manually add them.