The compressing method is still confusing. But as suggested by @Loneshade, i obtained the full overhead mountain code for a 225*225 map. The main point is that 0.18 version can still congnize <roofGrid><roofs> tag though a normal savefiles uses <roofGrid><roofsDeflate> instead. So you can just open a normal 0.18 savefile and replace its <roofGrid><roofsDeflate> tags and content with <roofGrid><roofs> tags and content (repeating RCpEKkQq for 225*225/3 times, wow), and load the game, then save the game. and open the savefile again, you shall see it is updated automatically to the code below:
<roofGrid>
<roofsDeflate>
7cIxDQAAAAKgPuawfyZbeDFoCgAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAABwNg==
</roofsDeflate>
</roofGrid>
<roofGrid>
<roofsDeflate>
7cIxDQAAAAKgPuawfyZbeDFoCgAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAABwNg==
</roofsDeflate>
</roofGrid>