difference between cars and buidlings

If the image is not good enough to clearly distinguish between the two, then the image is not good enough for tracing buildings. Don't use it.

Could you post a screen shot of a confusing image or even the map url or co-ordinates so we can have look. Usually something in the image will give a size reference and cars and trucks will be on a road and buldings will be beside the roads

The simplest way is to go and have a look at the area you are mapping to see what these objects look like from the ground by doing a site survey. However, that is probably not a practical proposition for every location. So you could start by looking at different aerial images of the same location to see if you can identify changes in the images. Vehicles are more likely to change positions than buildings are. Different images might also mean that you are able to view the same area at different times of the day and the different shadows that objects cast may give you clues about the 3 dimensional shape of the object. So you might be able to tell the difference between a shipping container sitting on the deck of a truck compared to one on the ground that is being used as a building. You might be able to see the container on the truck is raised off the ground, for example.

However, the biggest clue is probably size. Most vehicles are about 2 to 3 metres wide and perhaps 4 to 6 metres long, although a large bus or truck could be 3.5 metres wide and 25 metres long. Also shipping containers have standard sizes, so at the same zoom level vehicles will appear to be about the same size in different images. So by comparing images you may be able to distinguish vehicles from buildings. So, if you are stuck, try finding an aerial image of an area that you know contains both buildings and vehicles of the sort you think are contained in your mystery image. See what vehicles and buildings look like at the same zoom level in both images. It helps if both images are of the same quality, too. If you cannot tell what objects are from your known image, then you will know that you won't be able to tell that from the mystery image.

The context of an image also can provide clues, especially if you can see the white lines of a road or parking lot. This will give you an idea about how big your vehicles should be. White lines used in road-marking, which are often about 100 mm wide, can also give you clues about an image's quality. If you cannot clearly see the white lines from known road markings then the image might not be sharp enough to distinguish between vehicles and small rectangular buildings. If your image does not have road markings then look for other similar sized objects such as fence posts, fencelines or power poles.

Also at a zoom level of 19 the wire frame lines in many map editors will produce a mapped object's outline that will appear to be about 100 mm wide, while the enhanced highlighting will appear to be 1 to 2 metres wide. This will make it challenging to accurately draw vehicle sized objects in some editors at a zoom level of 19. However, most buildings, apart from a shipping container building, stand alone toilet or equipment shelter should be large enough to draw