General comments:

The paper by Blanch et al. entitled “Fixed photogrammetric systems for natural hazard monitoring with high spatio-temporal resolution” presents the design and construction of photogrammetric systems for high spatial-temporal resolution rockfall monitoring in two study areas in Spain. The monitoring systems have been installed in front of rockfall active cliffs and have been in operation for over a year. The authors provided in-depth information on the components, assembly instructions, and programming codes required to build the photogrammetric monitoring system with an implementation in real scenarios.

Overall, this is an appropriate subject area for NHESS journal, and the developed workflow, accessible to researchers from different disciplines, could help data collection and the 3D change-detection surveys, fundamental in terms of risk monitoring and prevention perspective. The manuscript is well-written and developed even for inexperienced readers. However, I believe that minor improvements can be made, for example, adding some aspects related to data post-processing and error assessment of photogrammetry outputs. Moreover, it is also required to add some details of previous works (only referenced in the manuscript) to have a complete understanding of the whole research. This work can be interesting and useful for the scientific community with some improvements.

Specific comments:

• Results:
  • Line 285-295: It could be useful for the reader to have maps of the study areas with the location of the camera’s network used for monitoring because the mere reference to previous work (Blanch et al., 2020) is not sufficient for the manuscript to have its own independence in understanding the carried-out study.
  • Line 299: Please briefly explain the workflow and acronym. The citation alone is not sufficient to ensure an understanding of the work done independently of previous work. In this way, the reader can better understand the study.
  • Line 303: Where are the GCPs located in the study area (a figure could be added about this)? Is it a network of fixed points? Did you use also independent Check Points (CPs) in the photogrammetric workflow? They are fundamental to understanding the errors and accuracy of the point cloud.
  • Has the problem of co-registration of point clouds been considered in the multi-temporal 3D surveys? This problem could affect the measure of the minimum change detection value between multi-temporal point clouds.
• Conclusions: could be expanded with more details to emphasize the innovative and useful points of the work done.

Typing errors

• Line 298: 2 cm/pixel or 2 cm pixel^-1
• Line 301: error of 0.23 m?? (units of measurement are missing)
• Line 302: what does "Mio points" stand for?
• Line 302: 6000 points/m² or 6000 points m^-2

In their manuscript, Blanch et al. present a detailed description of how to construct and operate cost-efficient photogrammetric systems, essentially a set of synchronized high-resolution time-lapse cameras. Using data from two study sites in Spain, they show that their synchronized setups of cameras attain similar accuracies in 3D change detection monitoring as terrestrial LiDAR, with the advantage of cost efficiency and higher temporal resolution. Their primary aim is to enable other researchers to set up similar systems. For this, the authors provide details on the components used and tested for their setups, give assembly instructions, and share the programming code needed for the operation.

The manuscript is well very written and clearly merits publication in NHESS as it facilitates the installation of low-cost monitoring systems by a larger community. Their setup can be built with a few hundred Euros and is applicable to monitoring of a wide range of environmental processes. Before acceptable for publication, the authors need to address a number of issues that I will outline below. As these are only minor points (see below and attached PDF), I recommend “minor revisions”.

Kind regards

General comments

• Chapter 2: In this chapter the authors provide a great amount of detail on the components used in their setup, on the assemblage and functionality of the system and the proposed workflow, which allows the reader (also the non-expert) to follow in detail. What I am missing here are details on the processing of the images in the next steps. Surely, the aim of the authors is to provide instructions for setting up a photogrammetric system, not going into details on the generation of digital topography from the images acquired. However, as the Chapter 3 looks at the accuracies of the models that can be attained, it would be nice to at least provide some short paragraph on the workflow and settings used for the generation of 3D models. The same applies for the M3C2 algorithm; here the authors point to the fact that the settings have a crucial influence on the results – providing those details for the examples would be very much appreciated.
• L212-214 and L239-244: This only becomes clear later in the text. Maybe the authors could also indicate at this point that the idea of this second relay is to trigger it by running a code in the case that the system should not shut down automatically.
• L439-455: As the paper is rather focusing on the technical aspects of the setup, this section comes a bit as a surprise. While I think some of the points are valid, they would require an in-depth analysis. I would recommend leaving out this section here and focus on the discussion.