Forecasted values on map change depending on zoom level

  • I've noticed that the forecasted values displayed on the maps will change as you zoom in or out. For temperature it may only be a couple of degrees. However, for cloud coverage, it can change by 10% (coverage) or more.

    What is happening here? The only thing I can think of is that the app is sampling a region around the location and as you zoom in and out the region size changes.

    I addition, when you change the zoom level, the value is initially way off and then after a second our two it corrects itself to the value that changes somewhat with the zoom level.

  • Sailor Moderator

  • Thanks, I just read though the other reports of it.

    If this is not a bug, it is at least a suboptimal design. Surely you would not want this to happen if it could be avoided. I never know what the actual value is and thus cannot fully trust the map reading other than to give a sort of estimate.

    It does sound like it is due to some sort of averaging over an area. I guess the point value for that location is not readily available.

    I will also say in Windy's defense that in practice it probably doesn't make any difference. Surely the error bars on most of the forecasted values are as large or larger than the variability while zooming. It just offends my engineering sense of precision.

  • Sailor Moderator

    Yes in practice it is a small issue. As you know forecast data from weather models are supplied by GRIB files along a grid at model resolution. Then Windy interpolate between the grid figures to give a continuous image. To have an idea of what spatial accuracy this grid can offer, just look at Precipitation type layer. Here there is no interpolation between precipitation types. I have added points 1 and 2 at 9 km distance which is the resolution of ECMWF model.


    So the interpolation between several points of the grid gives probably this apparent inaccuracy when you change the zoom level. Is it important? No, because interpolation is just a way to smoother discontinuous figures.
    Worse when you are in mountains with a 23 km resolution model like GFS can you imagine that temperatures at surface are accurate?
    BTW Precipitation type layer no longer seems to work with GFS.

  • Thanks for the additional info. Possibly I’m starting to understand the issue. When viewing the map while zoomed out, is the grib file sent to the app at a lower resolution? If the grib file’s resolution varies with the map zoom level then I can totally see how the interpolation between points can change with zoom.

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    Ive noticed this zoom dependence as well. At btschumy mentions its troubling because it unclear which value to trust, especially when there is a large discrepancy. I've seen over a 3x variation in forecasted PM 2.5 where it means the difference between Moderate and straight up Unhealthy air quality. Can this be interpreted as meaning the uncertainty in the forecast is even greater than this 3x discrepancy? This seems reasonable for a forecast days out but I've seen this discrepancy for the current time.

    btschumy has a good point on grib file resolution possibly being sent at a lower resolution when zoomed further out. Can you confirm this is the case? Otherwise, I'm struggling to understand why the interpolated value would vary with zoom level. The only other explanation I can think of is if the coordinates of the chosen point are rounded more as you zoom out. For example, at a close zoom, say the interpolation is calculated by rounding the coordinate to the nearest thousandth of a degree. But zoomed further out the interpolation is calculated by rounding to the nearest tenth of a degree. In this case the computed forecast location could vary by a few miles which could make a huge difference.

  • Sailor Moderator

    @userJeff said in Forecasted values on map change depending on zoom level:

    I've seen over a 3x variation in forecasted PM 2.5

    I’ve never seen such a variation in forecast figures at same date and same location when zooming in or zooming out. Just about 10% can be noticed. So please send screenshots showing clearly what you get when zooming.

    @userJeff said in Forecasted values on map change depending on zoom level:

    grib file resolution possibly being sent at a lower resolution when zoomed further out

    What do you mean? Grib files are sent from Weather Center to Windy. How then by changing your zoom level on your screen could change the resolution of the grib file ??

    For whatever reason the number in the weather picker shows a variation depending on the zoom level, is it so amazing? On Windy you can switch from a map covering 180° longitude, ie half of the globe, which is about 14000km at latitude 45 ° .... to a surface covering 55km when zooming in to the maximum. As already said, interpolation is just a way to smoother discontinuous figures. A 10% variation of interpolated figures should not be a problem, as by definition, interpolated figures are not accurate.

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    @idefix37 I'm unable to reproduce the 3 fold discrepancy I was seeing last week. But here are some screenshots showing a 2x descrepancy for the same location, which is still pretty significant, especially for a forecast for the current time. This is a pretty big difference in visibility when hiking in the mountains.




    I'm not exactly sure what it would mean for the grib file to be sent at lower resolution. It was previously mentioned as an explanation for why the interpolated value would be different at different zoom levels. Something seems to be more course at higher zoom levels.

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