Choosing good OpenDroneMap parameters
Introduction
I had an interesting question recently at a workshop: “What parameters do you use for OpenDroneMap?” Now, OpenDroneMap has a lot of configurability, lots of different parameters, and it can be difficult to sift through to find the right parameters for your dataset and use case. That said, the defaults tend to work pretty well for many projects, so I suspect (and hope) there are a lot of users who never have to worry much about these.
The easiest way to proceed, is to use some of the pre-built defaults in WebODM. These drop downs let you take advantage of the combination of a few different settings abstracted away for convenience, whether settings for processing Multispectral data, doing a Fast Orthophoto, flying over Buildings or Forest, etc.
You can also save your own custom settings. You will see at the bottom of this list “Steve’s Default”. This has a lot of the settings I commonly tweak from defaults.
Back to the question at hand: what parameters do I change and why? I’ll talk about 7 parameters that I regularly or occasionally change.
The Parameters
Model Detail
Occasionally we require a little more detail (sometimes we also want less!) in our 3D models from OpenDroneMap. Mesh octree depth is one of the parameters that helps control this. A higher number gives us higher detail. But, there are limits to what makes sense to set for this. I usually don’t go any higher than 11 or maybe 12.
Elevation Models
DTM/DSM
Often with a dataset, I want to calculate a terrain model (DTM) or surface model (DSM) or both as part of the products. To ensure these calculate, we set the DTM and DSM flags. The larger category for DTM and DSM is Digital Elevation Model, or DEM. All flags that affect settings for both DTM and DSM are named accordingly.
Ignore GSD
OpenDroneMap often does a good job guessing what resolution our orthophoto and DEMs should be. But it can be useful to specify this and override the calculations if they aren’t correct. ignore-gsd is useful for this.
DEM Resolution
DEM Resolution applies to both DTMs and DSMs. A criterion that is useful to follow for this setting is 1/4th the orthophoto resolution. So, if you flew the orthophoto at a height that gives you 1cm resolution ortho imagery, your dem-resolution should probably be 4cm.
Depthmaps
Depthmap resolution
A related concept is depthmap resolution. Depthmaps can be thought of as little elevation models from the perspective of each of the image pairs. The resolution here is set in image space, not geographic coordinates. For Bayer style cameras (most cameras), aim for no more than 1/2 the linear resolution of the data. So if your data are 6000×4000 pixels, you don’t want a depthmap value greater than 3000.
That said, usually, 1/4 is a better, less noisy value, and depthmap calculations can be very computationally expensive. I rarely set this above 1024 pixels.
Camera Lens Type
I saved the best for last here. So, if you’ve made it this far in the blog post, this is the most important tip. In 2019, OpenSfM, our underlying Structure from Motion library, introduced the Brown-Conrady camera model as an option. The default for camera type is auto, which usually results in the use of a perspective camera, but Brown-Conrady is much better. Set your camera-lens to brown, and you will get much better results for most datasets. If it throws an error (which does happen with some images), just switch it back to auto and rerun. Brown will be a default in the near future.
