離線地圖、放大和縮小。
從 4.3 版本開始可用
允許顯示未包含在已導入地圖或下載的離線地圖中的縮放等級。
當地圖提供者/伺服器僅提供有限的縮放等級下載時，例如 2-14，但你需要使用 15-19 的縮放等級，這會很有用。通常提供者不鼓勵下載超過 16 級的內容。
此外，有時你可能只有一個縮放等級的地圖。這通常發生在將向量 GeoPDF 轉換為光柵地圖（mbtiles）時。你會得到一個僅有縮放等級 8 的地圖，這確實很受限，除非你可以進行放大和縮小。
以下是其中一個轉換的範例：GeoPDF to raster mbtiles*, only zoom level 8 is present in the translated mbtiles:
As you can see it is approximately 23km per centimeter on a map. Without overzoom or underzoom, you’d see nothing on the map if you zoom in or out.
With overzoom, you’ll see a little bit pixelated, but still usable map. Here is the same map, for zoom 11:
About 3km per a map centimeter, a big difference.
For the underzoom. Here is zoom level 5:
Still better than having blank tiles? Arrow in the above image is to remind you that you can tap here when the map is not in the centering mode and focus on the map area, if you are asking yourself “where is the map” :)
With overzoom capabilities you can now decide volunturaly to download faster/less tiles from the map servers, with the concious trade-off for some pixelation on higher, undownloaded zoom levels. That’s exactly higher zoom levels (17-19) that represent the biggest performance challenge to the map servers. Be a good citizen.
Still, when selecting zoom levels for download, check they have the detail you need. E.g. smaller street names might be missing on lower zoom levels, overzoom is not going to fix this.
* Translating GeoPDF pdf file to mbtiles is as easy as this:
gdal_translate geopdf.pdf map.mbtiles
你可以 Google “install gdal on mac” 來查看如何安裝 gdal，這非常簡單。
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