Okok now I understand. Thank you for your help :-) 2010/4/22 William <[email protected]>
> On Apr 20, 11:07 pm, Andrew Leach <[email protected]> > wrote: > > > > At the equator, 1 degree is 111320m, which means that 0.000001deg is > > around 11cm. Seven decimal places allows you to place a marker to an > > accuracy of around a centimetre, which can't be shown on a map. > > > > if there's a google map with pixel dimensions smaller than this > resolution of 0.000001 degrees, then we wouldn't be able to locate > every pixel. That way we could show a map where 6 digits was not > enough. > > Let's look at the typical maximum zoom of 20 on satellite maps, with > pixel dimensions of 0.00000134 degrees at the equator. This is ok at > the equator, but as we move towards the poles, the scale of the > mercator map increases according to the secant of the latitude. As we > go north, everything appears larger than it really is (like we are > zooming into the map, but without changing the zoom level). > > This causes a problem on zoom level 20 where secant(latitude) > 1.34, > which is anywhere greater than 41.7 degrees. > > For example Seattle, which has orthophoto aerial imagery of 10cm > resolution. This is more precise than the 6 digit resolution of > 11cm. If you were using google maps in Seattle to trace building > outlines at maximum zoom, you would need 7 digits of precision for the > latitude to be sure you could locate every pixel on the map. But 6 > digits of precision would still be ok for the longitude. > > There are some cities with a greater zoom level of 21 for satellite > maps, like Tokyo and Sydney. They would need 7 digits of precision to > maintain the accuracy of the source imagery. > > ... > > -- > You received this message because you are subscribed to the Google Groups > "Google Maps API" group. > To post to this group, send email to [email protected]. > To unsubscribe from this group, send email to > [email protected]<google-maps-api%[email protected]> > . > For more options, visit this group at > http://groups.google.com/group/google-maps-api?hl=en. > > -- You received this message because you are subscribed to the Google Groups "Google Maps API" group. To post to this group, send email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/google-maps-api?hl=en.
