Hi,
No problem.
Each object list is basically a pointer to objects with little extra info. Maybe one int to count objects in each list. Thats about 8 bytes.
8 bytes*1000 = 8kb... not much mem at all...

8*50000=400kb... average PC has way larger memory. Consider in this case your 50000*50000 map will consume at least 1 byte/entry... thats about 2.5gb memory...

Now, if you have that size of maps, dont try to load all the map in one time. Brek it into 256x256 chunks and keep 9 chunks (central and the 8 around) loaded at a time.

Now, each sprite HAS a central point. Its exactly where the sprite is located in the world. Say a tree is located in your world at position (32.3f, 24.5f). You place it in the list number 24. When you process the list, you must consider an additonal extent. If you have to process rows from 100 to 120, then consider processing rows from 90 to 130 so objects may be processed.

Consider that you are rendering 40 rows. But you are saving 960.

There are some other considerations when this is useful, particularly when processing lights and blockers for real time shadow.

My old game engine:
http://www.spritekin.com/kengine/screenshots.htm

Luck!
Guimo

Why waste memory on a tile struct? You can just have a 2D array w/ an int value to represent tile and base tile type by where it the tile is on the tileset, so you group the blocking tiles, non blocking, water, etc.

Edit: Wait, did you actually like draw the background of the map or did you work off a tileset? If you just drew it, you still don't need a tile struct, just need to store each tiles value whether its swimming blocking nonblocking etc......