Erosion pins are difficult to measure and access and seem most applicable for confined areas. Animals and people can sometimes damage them if left in place. They are also sensitive to surface disturbances from other outside sources, such as trees or limbs falling. I have used washers before, but the washer itself will affect the rainfall impact and if there is surface flow around the pin itself. Some soils leave pedestals of even small rocks, which the washer may also do.  If the erosion pins are graduated such as like an engineering scale, that might work.  If you colorcoded the measuring pins with different colors every mm or in some recognizable sequence, perhaps you might be able to read each pin from some distance and get to the nearest mm.  But certainly reading erosion pins tends to be a time consuming challenge..  

In the sediment from a small ephemeral gully study (researchgate list 2007), we were measuring sediment accumulation in a small fabric dam and used a rod, laser level to measure readings, but we were able to walk on the surface and reach out to the measurement locations set up by border pins with little disturbance on placing the measurement horizontal rod used for site locations where the vertical rod was used to read elevations from laser level. If the area of the pins could be accessed, the laser level with rod for elevations of each pin may be useful.  

If you know someone with ground based LiDAR system, I have been wondering if that could be applied and would suffice for the detail needed to measure small surface changes over time by incrementally mapping the surface contours at a fine local scale with elevation benchmark.  I don't know the costs, practicality of this.  

They used to make systems that could read water surfaces in streams from perhaps a bridge by beaming radar to water surface.  In addition, doppler systems are now used to measure channel form and stream velocity for stream measures, so perhaps that technology might be applied.  I dont know if this equipment might be used in designing a non-pin measurement system. I would suggest finding someone with this type equipment who is willing to spend a day with you to see if it might work for your needs.

In the 2007 report mentioned, we used a Criterion Survey Laser to define boundaries of a small gully.  The laser is a survey instrument that bounces the laser off a surface, such as ground, tree, etc. and give distances and other measurements.  Perhaps that could be used in some way similar to LiDAR, but it may be slow or difficult to implement.  

Bottom line, measurement pins are relatively inexpensive to install, but have challenges to read with precision without damaging the surface, and washers in some circumstances may work fine if pedastaling is not an issue.  The research depends alot on that pin being and remaining undisturbed if left in place.  Having pins with recognizable graduations may be helpful in reading with some speed.  Perhaps for small areas of evaluation, even a high resolution photograph might be taken of the pins during each measurement trip and you could figure out a way to detect and measure heights from the photos, as well as keep a visual record of plot changes over time.

Dear colleague, thank you very much for your question. We used of a graduated tape.

Best regards

Vincent

Dear colleagues

Thank you very much for your useful answers.

Would you please add some photos of your graduated pins or taps? or you may have some results and papers using those equipment?

Regards

Please note that in our study we used pins inserted in gully banks and thus do not suffer from animal trampling, ... to the same extent that pins would suffer when inserted vertically on the soil surface.

These are very unexpensive, easy to implement but do not give the accuracy of laser based or RTK systems.

We measured erosion in railway cuts using pins and a microbridge to avoid the influence of the pins in erosion. You can find out how we worked in this paper.

I wish it be interesting for you.

Dear colleagues

Thank you very much for your useful answers.

It was interesting for me.

Regards

I do not have an answer, but I appreciated the suggestions and the paper sent by Joaquin Navarro.