A Mercator projection for Nearmap imagery
In testing Nearmap’s not-for-profits’ plan, Russell Rigby of NSW reported that after analysis;
- The Google Mercator units aren’t even close to metres so current Nearmap imagery cannot be imported straight into OCAD at a meaningful scale.
- The OCAD transformation [via registration of image corners to OCAD map] from Google Mercator to Map Grid of Australia is good, but needs testing in more areas to be sure.
Following further investigation by Russell and myself we determined that the EPSG code 3785 referenced by Nearmap was not present in OCAD’s co-ordinate system range. Further detective work showed that OCAD contains the original version of the Google Mercator projection (EPSG 900913) but it is no longer officially in use. Not that a cursory search of the spatial world would tell you that. Fortunately Russell came across this post in Alastair Aitcheson’s spatial blog.
Popular Visualisation CRS / Mercator
I advised OCAD hq of the issue along with the background we discovered and they got onto the matter immediately. Their recent service release 126.96.36.1991 of 2013-09-04 contains the co-ordinate system referenced by Nearmap images available under the latter’s Basic plan. In the OCAD co-ordinate system list it is named Popular Visualisation CRS / Mercator (EPSG 3785). Note this addition is not documented in the release notes.
Images previously downloaded via Hypertiles
If you downloaded images via Hypertiles before Nearmap closed off the free service, then you should find those images are georeferenced. Thus if used as a background image, the images will register nicely with a georeferenced map.
You can check the co-ordinate system of such images by opening the .vrt file that is in each Hypertiles data set. The first few lines show;
[quote] <VRTDataset rasterXSize=”75677″ rasterYSize=”113090″>
However, whichever you selected when running Hypertiles, OCAD should convert nicely.
Russell Rigby on trialling Nearmap’s Basic plan
(Nearmap’s Basic plan offered to orienteering appears to be the same as, or very similar to, their Photomaps Standard Plan).
There is no equivalent of Hypertiles now, where you can nominate the area and resolution level that you want ( at least at the subscription level that is in any way likely).
The current subscription model has a download limit (50MB/month basic, next level 250MB/month). Any image displayed on the screen is included in downloads, to the extent that Nearmap suggests reducing the size of the browser window as a way of reducing the downloads. Panning or zooming adds to the download, as does viewing an image sequence. Using the screen tools (measuring, area, lat/long etc) does not add to downloads as long as the image is not moved.
There are two choices when saving an image. The first is to effectively capture the screen, the second is to define a box to download at high resolution. Both options can generate a world file, in the format attached to the image I sent previously. The only indication of the size & resolution of the resulting image file comes after the download has been requested and performed, so working on a tight download budget could be difficult.
The jgw is the only world-file format – there is no equivalent of the vrt file. Nearmap help suggests loading the output files into a GIS system (ESRI, Mapinfo etc) to work further with the image.
In the test data area Level 17 corresponds to approx 1m pixel size (1.19 “units”), and each change in level corresponds to a factor of 2 eg level 18 == 0.5m pixel. The only way I can see to control download size is to measure the max dimension of the area I want, & do a quick calculation of which scaling level will give between 3k & 6k pixels along that dimension, or I get to level 21 (approx 6cm) which is the smallest pixel size available.
The quality of the imagery is very high, and for capital city areas, very frequent. Regional coverage is less frequent (3-6 months for Newcastle), & is patchy and one-off for other coastal & inland towns in NSW. Many towns are not covered, or are only photographed in flood time! Some areas I have looked at have more recent coverage on Google or Bing, but usually not as high resolution.
The terms & conditions for the licence say that the first 2 months have unlimited downloads.