OMCircle basically does what your are doing - it finds the points at
different angles at the same distance from the center, and then connects
those points as a polygon.
I'm not sure why your handle would show up in a different place than the
circle edge, especially if the N/S points are lining up, but I don't
understand your algorithm for finding the E/W handle (distance from the
center to that point).
- Don
Christopher Franklin wrote:
> Hi Don,
>
> Thanks for replying so quickly.
>
> Don Dietrick wrote:
>
>> Hi Chris,
>>
>> Christopher Franklin wrote:
>>> Greetings,
>>>
>>> I am working on a project that requires a user to select an area on
>>> the map by using a circle. The user clicks to select the center and
>>> then again to select the edge (radius). I am using an OMCircle
>>> because it exhibits the proper behavior for a circle on a globe.
>>>
>>> I have noticed a problem with the OMCircle though. After drawing it
>>> using the provided center and radius I am adding a graphic to
>>> represent the center and another to represent the rightmost edge of
>>> the circle. When the circle is drawn on or near the equator
>>> everything appears correct. However, if the circle is drawn north or
>>> south there is a noticeable distortion in how the circle and the edge
>>> handle are drawn. I have done some experimentation and discovered
>>> that the handle is being drawn in the correct location, but the edge
>>> of the circle is not. The further away from the equator the greater
>>> the discrepancy between my handle location and the OMCircle edge. If,
>>> however, I move the handle to the top of the circle it is always
>>> drawn in the correct location. For usability purposes I would like
>>> the handle to be on the right hand side! Is there a way to fix this?
>>> If not, I guess they will have to deal with the handle being on the
>>> top or bottom of the circle...
>>
>> How are you creating the handle location, and what are its location
>> parameters? How do you know the handle is in the right place?
> The handle location is being generated by taking the current position of
> the center in Lat/Lon and adding the circle radius to the Longitude. To
> generate the the handle position on the "top" of the circle I instead
> added the radius to the Latitude.
>
> To determine that the handle was in the proper location took a bit of
> experimentation. I have the ability to produce multiple circles at a
> time. By experimentation I determined that by clicking on the center of
> the circle and my artificial handle I was able to create the same size
> of circle. If I click on the center and then put the radius on the
> displayed circle edge I get a new circle with the edge handle being
> drawn in the location of my second mouse click, but the drawn edge of
> the circle is about 1 degree further out. The distance of the distortion
> increases the further away from the equator I place the circle. The
> problem persists at lower levels of zoom, although the ratio of
> distances stays the same, just the scale changes. For instance at a 100k
> zoom level the circle edge would be drawn about 1k further out than it
> should be.
>
> I hope this helps make the problem clearer... it is easier to see than
> to describe in words!
>>
>> OMCircles created with distance radius shouldn't appear as circles in
>> certain projections when you move away from the equator. Pixel
>> distance from the center will vary along the circle edge for the same
>> distance (depends on latitude, angle on edge from center).
> I realize this. In fact, the problem I am trying to describe is actually
> drawing the circle as far more circular than it actually should be. The
> handle I am laying down shows that the circle is slightly off from where
> it should be. I am going to go out on a limb here and hypothesize that
> what it looks like is the 800m difference between 1 degree lat and 1
> degree lon is not being taken into account. Feel free to slap me if I am
> wrong on that one!
>>
>>>
>>> Another problem I would like to bring up is when zoomed in to a small
>>> area (~1m scale) the OMCircle draws as a polygon approximation of a
>>> circle. Is there a way to increase the number of segments being drawn
>>> at that low of scale?
>>>
>>
>> OMCircles let you set the number of segments used to represent the
>> circle, but you might be seeing a precision problem that occurs from
>> using floats in the OMCircle. The next version of OpenMap will
>> address the precision problem.
>>
>> Hope this helps,
>>
>> Don
>>
>> --
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>
> I hope this clarifies things a little for you. If not let me know and I
> can either clarify more or post some code if that would help.
>
> Thanks,
>
> Chris
>
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Received on Tue Jul 29 2008 - 07:52:28 EDT