What is... (a densified point cloud? an orthomosaic? etc.)

   

What is a densified point cloud?

The densified point cloud is a set of 3D points that reconstruct the model. The X,Y,Z position and the color information is stored for each point of the densified point cloud.

The densified point cloud is computed based on the automatic tie points (ATPs) of step 1. Initial Processing and it provides a very accurate background for distance, surface and volume measurements.

Point_Cloud.png

What is a 3D textured mesh?

The 3D textured mesh is a representation of the shape of the model that consists of vertices, edges, faces and the texture from the images that is projected on it.

It is useful to present and visualize the model, share it and upload it to online platforms such as Sketchfab. It is intended to look nice more than to be accurate, so it is not recommended to use it for measurements.

3D_Textured_Mesh.png

What is an LOD mesh?

The LOD (Level Of Details) mesh is a representation of the 3D textured mesh that contains multiple levels of detail, decreasing the complexity of the model as it is divided into more levels. Fewer details are available as zooming out in the model.

The LOD mesh is useful for visualization on the web and from different types of devices, such as desktop PCs, tablets, smartphones, when fast loading is needed. It is the ideal output for a smooth and efficient navigation on online viewers.

 
Note: The LOD mesh cannot be visualized in Pix4D Desktop. It can be exported and visualized in other software: Skyline TerraBuilder, SuperMap or in web viewers, such as ArcGIS Online.

For step-by-step instructions about how to export the LOD mesh: 212218463.

What is a polyline?

A polyline is a continuous line composed of one or more sub-lines. It is created by specifying the vertices of each line.

polyline.png

Pix4D gives the ability to draw 3D polylines, where each vertex has three coordinates. Polylines are typically used to measure distances in the model.

For step-by-step instructions about how to draw a new polyline with the rayCloud: 202560309.
For more information about projected 2D length and terrain 3D length of polylines: 202559819.

What is an orthomosaic?

The orthomosaic is a 2D map. Each point contains X, Y and color information. The orthomosaic has uniform scale and can be used for 2D measurements (distance, surface). It corrects the following problems of the input images:

  • Perspective of the camera.
  • Different scale based on the distance that each point of the object / ground has from the camera.

The orthomosaic generation is different from photo stitching. For more information: 202558869.

orthomosaic.png

What is a reflectance map?

The reflectance map is mainly used when the input is multispectral / thermal imagery. For more information about the reflectance map vs orthomosaic: 202739409.

For each band, Pix4D produces one reflectance map. The goal is to properly assess the reflectance for a particular feature based on the pixel value in the images. This pixel value is influenced by many factors, including but not limited to:

  • Incoming light.
  • ISO.
  • Aperture.
  • Shutter speed.
  • Sensor response and optical system.
  • Vignetting.

Pix4Dmapper, therefore, uses the camera positions and the 3D model to take these different factors into account and produce an accurate reflectance map. To obtain representative results, it is important to have the right equipment. For Pix4D's recommendations concerning camera requirements: 204894705.

 
Note: In order to produce accurate reflectance maps, multispectral imagery can be radiometrically calibrated and corrected by Pix4D. For more information: 206494883, 202559509.

What is a DSM?

The digital surface model (DSM) is a 2.5 D model of the mapped area. It can be exported as two different types of files:

  • Raster GeoTIFF.
  • Vector point cloud (.xyz, .las, .laz).

Each pixel of the raster geotiff file and each point of the vector point cloud contain (X,Y,Z) information. They do not contain color information.

dsm.jpg

For each (X,Y) position, the DSM has only 1 Z value (the altitude of the highest point for this (X,Y) position). This is why it is considered to be a 2.5D model.

 
Example: The edge E in the following image has 2 vertices with different altitude values (Z2>Z1). In the DSM this edge will only be shown as the point (X,Y,Z2).
 
2.5D_house.jpg

 

2.5D_from_top.jpg

House - Side view House - Top view - DSM

For more information about how to visualize the raster DSM file: 205084685.

What is a DTM?

 
Video: Watch the instructional video related to DTM: here.

The digital terrain model (DTM) is a 2.5 D model of the mapped area after filtering out the objects, like buildings. It can be exported in GeoTIFF raster file format.

Each pixel of the raster file contains (X,Y,Z) information but they do not contain color information.

 

DTM_lausanne.png DSM_lausanne.png
DTM of the city center of Lausanne DSM of the city center of Lausanne

What is an index map?

Pix4D allows to work with several well-known indices such as NDVI or to create custom indices. For more information: 202560489.

To each index is associated an index map. For each pixel on this map, the value of the pixel is derived from the associated reflectance maps.

 
Example: Consider the custom index with formula (RED NIR) / (GREEN + 1) and a fixed pixel. Say the associated pixel in the reflectance maps has the following values:
  • Pixel value in the red reflectance map: 0.4.
  • Pixel value in the NIR reflectance map: 0.6.
  • Pixel value in the green reflectance map: 0.1.
The value of this pixel in the index map is (0.4 0.6) / (0.1 + 1) = 0.18.
For more information about how to generate index maps: 202560529.
 
index_map.png
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