Dealing with artifacts in Wilbur

[acrosome]

My current project is mapping a terraformed Venus- see my WIP in the world/regional forum. I found the MIT topo image- based on Magellan data- for the planet as a grayscale image and loaded it into Wilbur (my first time using Wilbur). I figured using Wilbur might be easier than the hand-drawn stuff I'm transcribing from the USGS topo map of the planet. But the MIT data has some odd artifacts:



This is one small area to give you an idea of what I'm talking about. My question is, as a new Wilbur user, Wilbur seems to be able to do a LOT of cool stuff, but I have no idea even where to begin. Hell, I'm currently working on how to set a sea level...

How can I get rid of those streaks at the left third of the image that Wilbur reads as higher elevations?

How I can extrapolate in a reasonable fashion into the blank spots in the data, like that bit in the upper left? (There are much larger ones elsewhere.)

The grayscale apparently maxed out too low as well- the higher elevations all look like perfect plateaus, as you can see. I'll have to deal with that somehow- probably by hand modifying from the satellite images or something. But that's lower on the priorities I think.

Or would it be better to manipulate the original grayscale image in GIMP or something?

Thanks, All.