Field Work

Field Work

Choosing your Camera and Lens

The following guidelines and recommendations will help you choose an optimal camera and lens for surveying work:

·         Use a high-resolution camera. It is recommended to use a camera with at least an 18-megapixel resolution. Typically, an 18-megapixel camera provides a 1 to 2-centimeter (3 to 6 hundredth of a foot) measurement accuracy at a distance of about 100 to 150 meters (110 to 160 yards, 330 to 480 feet).

·         Use a camera with a mechanical shutter. It is recommended to use a camera with a mechanical shutter, as it will allow you to take images while the drone is moving.  This results in a better utilization of battery resources allows covering larger areas in less time.

·         Use a lens with a fixed focal length. DatuBIM requires that the camera’s focal length will not change. Thus, it is strongly recommended to use a fixed focal length lens, i.e., a lens without an optical zoom.

 

Photographing the Survey Area

The following guidelines and best practices will help you photograph the survey area in an optimal way. Here you will find some general recommendations, as well as recommendations for taking oblique images and for taking vertical images.  Please refer to Best Practices for Drone Photography article as well.

 

General Recommendations:

·         Use a drone to take images using Datumate’s automatic flight planner DatuFly Professional if possible. This will increase the probability that images will be stitched into one cluster.

·         Make sure your images cover at least four ground control points (GCPs). Even though the minimum mathematical requirement to build a model is three GCPs, it is strongly recommended to use at least four. Thus, when control points are used, make sure that your images cover at least four with each one appearing in at least three images.

·         Set your camera to maximum resolution. Note that some camera models are preset to photograph in a resolution lower than their maximum resolution. Make sure your camera is set to the maximum resolution provided.

·         Make sure camera is set to save the GPS location of the images. This is essential for proper image processing.

·         Take your images in good and consistent lighting conditions. Take all images in similar lighting conditions, preferably at the same time of the day. Avoid photographing the survey area during different times of the day. This will minimize changes in shadings and lighting conditions that may degrade the performance of the automatic image processing algorithms employed by DatuBIM.

·         Maintain proper overlap. It is recommended to maintain at least a 75% front overlap and a 65% side overlap.

 

Marking GCP's in the Survey Area

This section describes the best practices for marking and measuring Ground Control Points based on the area to be measured.

General Guidelines:

The following guidelines and best practices will help you measure your control points in the survey area in an optimal way:

  1. Measure at least four control points in the survey area of a small project. It is very strongly recommended to measure and mark four or more control points. Measuring and marking more control points in additional images will improve the resiliency of the geo-referencing process, assuming that the measurement of the control points is accurate itself. You may measure point coordinates in either a local or a national grid. The software will allow you to work with three control points marked in two images each.
  1.  Measure as many control points as possible of large-scale areas. It is very important that you cover all safely reachable areas, especially on the perimeter of the area of interest.
  1. Choose clearly defined control points. Choose control points that have a high contrast to the area around it and which will be unambiguously and accurately identifiable in the images. Use available landmarks or deploy your own prominent objects or marks in the survey area if no appropriate landmarks are available.   Alternatively, paint your own marks in the area.   Good examples of both landmarks and painted marks are shown below.
Choosing Clearly Defined Control Points
 
                                    Painting your own marks            

  

                                                 

  1. Avoid measuring points on slanted surfaces. This is specifically important when using a Total Station with in reflector-less model.
  2. Accurately measure the position and elevation of the control points. Carefully measure the position and elevation of control points with an accuracy of better than 2 cm (0.79 inch). Remember, the accuracy of measuring the position and elevation of control points sets the accuracy baseline for all the measurements and drawings related to the images.



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