A couple of weeks ago, my area had snow all day and all night, and in the morning I took a ruler out and jammed it onto the deck. Six inches. I did the same with the railing, also six inches. It melted, but a week later it was eight inches deep. That's good enough for me, but it isn't quite accurate enough the National Weather Service. The NWS uses a network of volunteer weather observers across the country, around 8,700 of them, to report on local weather conditions, including snowfall and snow depth when the occasion calls for it. And they have rules for measuring snow. You should have the right place to measure snowfall, like a snow board, which should be measured and cleared often, but not too often. Snow depth is different, because it's the accumulation of snowfall and should be an average of measurements from different places, and will be affected by melting, compacting, and liquid precipitation.
Snow depth is like the sum of individual snowfalls, if one assumes no sublimation—snow turning into water vapor—or melting from the first snowfall until now. That assumption would almost always be wrong, of course. But if you suspend reality for a moment, the depth will still never exceed the sum of all snowfalls because snow is compressible. So, two 10.5-inch (27-centimeter) snowfalls may accumulate to a depth of only 17 inches (43 cm).
It’s the compressibility of snow that causes the greatest consternation with snowfall measurement. Snowfall is the amount of snow that accumulates during a given time, usually a 24-hour period. In a perfect world, this 24-hour period would end at midnight, but the vast majority of National Weather Service cooperative observers take their daily observation in the morning.
So it's a lot more complicated than just sticking a ruler into your yard. Learn how the NWS volunteers measure snowfall and snow depth at Atlas Obscura. You may be inspired to become a volunteer weather watcher yourself!