Vegetation Data: unit conversion for above-ground measurements

[brownag]

Following #373, where @phytoclast recommended providing SI conversions for some data elements. This is a good idea--but probably is more involved than we want soilDB to be in terms of how the raw data are being returned.

In general I think it is against our current ethos to be automatically changing things like this, but there are some instances where we do fix things (e.g. filling missing lowest horizon bottom depth, replacing NULL rock fragments with 0) that are clear-cut. I think for now I would prefer to stick to the data model and suggest folks propose a change to alternate units in the database. Now is the time to get stuff like this changed before we get too far with new Vegetation Plot standards, and while we have an forthcoming data model update...

Automatic conversion gets tricky... see questions below:

• Should soilDB provide support for unit conversion where the NASIS metadata indicate data should be stored in English units?

• If the user can opt in to the conversion it may be OK, but can we provide enough options that suit all user needs?

• An example is Vegetation Transect transectlength is stored in units of feet, not meters. Should we provide an alternative derived column for the converted columns? i.e. transectlength and transectlength_m

• Which functions would need to handle unit conversion?

• Which columns use non-SI units? And what units are used?

• How to handle rounding when converting to SI units? Should decimal places be retained or rounded to whole numbers? Rounding to whole numbers in e.g. feet to meters results in some loss of "information", but may not be meaningful loss in most applications. E.g. 10 points on a 100 ft transect would all result in unique values in meters. 100 points on a 100 foot transect would result in non-uniqueness

[phytoclast]

Fair enough. I agree that the fetch function probably should not auto convert units and obscure stored values. I agree it would be wiser to do this in a separate deliberate action. The rounding that I employ after conversions is to maintain the level of precision in the original field data. Plant height can be recorded to the nearest tenth of a foot, which means measurement in meters can only be considered to the nearest decimeter. Diameter can be recorded to the nearest tenth inch, meaning I can only read d-tape to the nearest cm.

I do already have tools in vegnasis package that consolidates the major variables and applies conversions so that data from multiple sources outside NASIS can conceivably be combined for analysis. Veg data collected by NPS, Natureserve, and state natural heritage programs are recorded in SI units, consistent with vegetation literature. Likewise, the calculations used in the background for scaling vegetation graphs are done more easily in SI because land area, stem diameter, and height are all in the same base units. Conversion or reversion to USC units for end user reporting should, therefore, be done after data is aggregated and summarized.