Could temperature in activities be logged with one decimal instead of just full degrees?

The value is only stored as an integer in degree C in the FIT file.

I have checked my display in Garmin Connect and my temperatures are showing in round numbers.  I have thought about this for a minute and about the only thing I can think of is the frequency that you have your 530 recording at.   See if you have your recording intervals set to 1 sec.

Mine is recording in F and is set to 1 sec.

Select Menu > Settings > System > Data Recording.

All data in the FIT activity file is recorded using SI units. The display will convert that value from a SI unit if you select a different unit. It will record temperature in degree C even if the display is set to show it in degree F.

I don't think the sensor is in fact accurate enough to give that level of precision. The number of significant figures displayed for a sensor reading has to reflect the accuracy and repeatability of the sensor. Otherwise those decimal numbers are just random numbers. 

Thats a solid argument, because no device in a group of drivers displays the same temperature. However, technically it should be possible to record one decimal.

https://developer.garmin.com/fit/protocol/ 

Just use an uint16 with scale 10 and offset 273.

Yes - this is really annoying.  A bunch of winter swimmers that have watches with thermometers like to check the temperatures during winter swimming.  You can see the decimals change in line with water temperatures when viewing the watch during the swim.  However, the data is rounded to integers in the Garmin app after upload  - which also means when automatically get the swim data from Garmin via Strava - we only get the whole numbers - our temp logging site is https://ozswims.co

We'd love to hear of some way around this. 

I concur, for exactly the same application (winter swimming). The sensor is actually precise enough when submerged in water, when compared to reference thermometer. So this bug is very limiting, and it has been there for ages. Could you please fix it?

Your trying to get accuracy where it doesn’t exist. The sensor wont that accurate in air. Basically adding another decimal place would be a random number. Why not 3 decimal places or 4? Sensors should only display the number of significant figures depending on the accuracy and precision of the measuring device. A sensor internal to a plastic enclosure won’t be accurate to air temp to justify decimal places perhaps half a degree intervals but no more.