20/12/2012

Reprojection issues


Reprojection issues

Someone asked me to show him the effects of reprojecting a cadaster map using QGIS from Cassini-Soldner to fit over a Google Map because he was unsure of certain things. Firstly, I explained that in QGIS, 'reprojection' is fundamentally a 2-step 'save as' move. After showing him how to do it, he agreed the QGIS way is comparatively easier than the proprietary GIS he uses. Secondly, I said to reproject a map to a Google Map, there are 2 options where many prefer Geographic Reference System e.g. WGS84 but I prefer Projected Reference System e.g. Goggle Mercator. Why? I am not clever in this field to technically justify a counter-argument like 'So what? I still can do it using WGS84. What's the big deal?' Well, my humble explanation is shown using the QGIS Coordinate Reference System selector: Note WGS 84 (I don't mean the WGS 84/Mercator-kind version) falls under the earth-preference Geographic Coordinate System whereas Cassini and Google Mercator fall under the paper-preference Projected Coordinate System (remember, the Geographic and Projected CS are fundamentally different in approach). As such, I suspect reprojection from Cassini to Google Mercator would be more accurate and reprojection from Cassini to a variation of WGS84 under the Projected Coordinate System ought to produce results but how much different I do not know:

Cassini map on-the-fly projection
When put off-the-fly projection, Cassini map disappears
Cassini map projection puts it west of Africa in the context of Google Map projection 
Cassini reprojected WGS84 map puts it in Peter Pan's Neverland
Cassini reprojected Google Mercator map Round 1 puts it above West Malaysia
Same map re-zoom to layer extent Round 2 brings it down to earth
Same map rezoom to layer extent Round 3 greatly improves map alignment
Memandangkan reprojection peta kadaster ke Google Maps boleh dicapai selepas beberapa cubaan, saya menganggap CPU, RAM dan juga kad grafik memberi kesan akan paparan peta ke Google Maps tetapi tidak pasti sama ada faiz fail yang besar ada hubungkait terus mengapa isu paparan tertimbul (Peta yang diguna mempunyai 1,161,254 polygon). Berilah saya komputer jenis CPU quad-core dengan RAM 8GB dan mungkin saya boleh beri maklumbalas yang lebih positif.

Since it is possible to fit the Reprojected cadaster Map to Google Maps after a few hiccups, I assume the CPU, RAM and even graphic card may have something to do causing the initial inability of QGIS to put things properly because the more I zoom in, the more accurate the map. On the other hand, I do not know whether it could be because the file size is too big for QGIS to handle (That map has 1,161,254 polygon features). Give me a quad-core CPU with 8GB RAM and I may give you a more positive answer.

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