[mapserver-dev] [mapserver-users] Detect featureless areas

Brent Fraser bfraser at geoanalytic.com
Sun Oct 7 08:50:55 PDT 2012

Interesting.  I was thinking of a similar concept ( "feature density"?) 
for use in label placement. For example when determining a location for 
a polygon label, select the location of minimum (or low) feature density 
(no roads, rivers etc).

Could be difficult.  Build a raster of line counts?  Then what?

Best Regards,
Brent Fraser

On 10/6/2012 3:42 PM, Stephen Woodbridge wrote:
> On 10/6/2012 4:33 PM, Ian Walberg wrote:
>> Stephen,
>> For an application that a user is zooming in I tend to agree I would not
>> care.
>> In our case we are displaying a rotating sequence of maps and want to
>> work out a way of not showing an image with an empty map on it.
> Well you might be able to do something along the lines of checking the 
> image size. I bet that if the image is below some threshold for a 
> given image size, that it has very little if any content in it. For an 
> 8 bit png image that is a 256x256 tile an empty tile is 103 bytes and 
> most of that is the header and the color lookup table.
> I think if you sample you images that you will find that all the 
> images that are empty are easy to detect by size.
> The tricky part to giving you a better answer is that you have not 
> defined "empty". If you mean a N x M image of one constant color, then 
> the above should work fine for your needs.
> -Steve W
>> Thanks
>> Ian
>> -----Original Message-----
>> From: Stephen Woodbridge [mailto:woodbri at swoodbridge.com]
>> Sent: Friday, October 05, 2012 6:00 PM
>> To: Ian Walberg
>> Cc: mapserver-users at lists.osgeo.org; mapserver-dev at lists.osgeo.org
>> Subject: Re: [mapserver-users] Detect featureless areas
>> On 10/5/2012 6:31 PM, Ian Walberg wrote:
>>> Hello list,
>>> We need to be able to detect when there are no features on a map and
>>> limit the extent so that the user does not zoom in any further if
>>> there is nothing to see.
>>> This could be for example no labels or no vector data for the given
>> extent.
>>> We are using php mapscript .
>>> Any ideas how we can do this?
>> Well on the one hand why do you care? Does this cause a problem in some
>> way? If the user wants to do something useless and stupid, then let him.
>> I would limit zooming based on scale or resolution.
>> The above would be my approach, but if you really want to do this, one
>> way to do this would be to setup the mapfile to do a query and then
>> query the layers you want to check for features based on the bbox of the
>> view and then check if you got any hits. But this can be potentially
>> much more expensive then drawing nothing in the case above.
>> It has been a while since I have used mapscript, but their might be a
>> return code that tells you if the layer drew anything (if not you might
>> want to write a ticket to add that.
>> You proposed method has the problem of scale dependencies, in the you
>> might have a situation where a some scale there is very little turned on
>> and it you stop zooming there, then you can never get to the scale where
>> other things get turned on. What you draw on any given scale is not
>> predictive of what you might draw at some other scale.
>> Think of a small island that is turned off because it is very small
>> until you get zoomed in. If you pan over the ocean to the vicinity of
>> the island and it is off, you would prevent zooming in to the point that
>> the it got turned on.
>> I would keep is simple, I think it will work better in the long run.
>> -Steve W
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