<div dir="ltr"><div>Yes, that was helpful, thanks. It has been reasonably easy to use "proj_get_area_of_use" in the C++ API</div><div>This is probably all that's needed for projected coordinate reference systems like EPSG:27700 where parameters are fixed (AFAIK).</div><div><br></div><div>In the end, I think the rest is probably a user or application-space problem. </div><div><div>Mercator can probably be dealt with by just throwing away any coordinates outside of what "proj_get_area_of_use" returns.</div><div></div></div><div><br></div><div>Seems the zones are hard-coded in the interrupted projections.</div><div>For cartographic purposes it'll probably be sufficient to build a clip mask from the projected nodes defined the source.</div><div>e.g. imoll.cpp:</div><div><br></div><div><p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Menlo;font-size-adjust:none;font-kerning:auto;font-variant-alternates:normal;font-variant-ligatures:normal;font-variant-numeric:normal;font-variant-east-asian:normal;font-feature-settings:normal;color:rgb(93,108,121)">/*</p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Menlo;font-size-adjust:none;font-kerning:auto;font-variant-alternates:normal;font-variant-ligatures:normal;font-variant-numeric:normal;font-variant-east-asian:normal;font-feature-settings:normal;color:rgb(93,108,121)"><span class="gmail-Apple-converted-space"> </span>Zones:</p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Menlo;font-size-adjust:none;font-kerning:auto;font-variant-alternates:normal;font-variant-ligatures:normal;font-variant-numeric:normal;font-variant-east-asian:normal;font-feature-settings:normal;color:rgba(0,0,0,0.85);min-height:14px"><br></p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Menlo;font-size-adjust:none;font-kerning:auto;font-variant-alternates:normal;font-variant-ligatures:normal;font-variant-numeric:normal;font-variant-east-asian:normal;font-feature-settings:normal;color:rgb(93,108,121)"><span class="gmail-Apple-converted-space"> </span>-180<span class="gmail-Apple-converted-space"> </span>-40 <span class="gmail-Apple-converted-space"> </span>180</p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Menlo;font-size-adjust:none;font-kerning:auto;font-variant-alternates:normal;font-variant-ligatures:normal;font-variant-numeric:normal;font-variant-east-asian:normal;font-feature-settings:normal;color:rgb(93,108,121)"><span class="gmail-Apple-converted-space"> </span>+--------------+-------------------------+</p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Menlo;font-size-adjust:none;font-kerning:auto;font-variant-alternates:normal;font-variant-ligatures:normal;font-variant-numeric:normal;font-variant-east-asian:normal;font-feature-settings:normal;color:rgb(93,108,121)"><span class="gmail-Apple-converted-space"> </span>|1 <span class="gmail-Apple-converted-space"> </span>|2<span class="gmail-Apple-converted-space"> </span>|</p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Menlo;font-size-adjust:none;font-kerning:auto;font-variant-alternates:normal;font-variant-ligatures:normal;font-variant-numeric:normal;font-variant-east-asian:normal;font-feature-settings:normal;color:rgb(93,108,121)"><span class="gmail-Apple-converted-space"> </span>|<span class="gmail-Apple-converted-space"> </span>| <span class="gmail-Apple-converted-space"> </span>|</p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Menlo;font-size-adjust:none;font-kerning:auto;font-variant-alternates:normal;font-variant-ligatures:normal;font-variant-numeric:normal;font-variant-east-asian:normal;font-feature-settings:normal;color:rgb(93,108,121)"><span class="gmail-Apple-converted-space"> </span>|<span class="gmail-Apple-converted-space"> </span>| <span class="gmail-Apple-converted-space"> </span>|</p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Menlo;font-size-adjust:none;font-kerning:auto;font-variant-alternates:normal;font-variant-ligatures:normal;font-variant-numeric:normal;font-variant-east-asian:normal;font-feature-settings:normal;color:rgb(93,108,121)"><span class="gmail-Apple-converted-space"> </span>|<span class="gmail-Apple-converted-space"> </span>| <span class="gmail-Apple-converted-space"> </span>|</p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Menlo;font-size-adjust:none;font-kerning:auto;font-variant-alternates:normal;font-variant-ligatures:normal;font-variant-numeric:normal;font-variant-east-asian:normal;font-feature-settings:normal;color:rgb(93,108,121)"><span class="gmail-Apple-converted-space"> </span>|<span class="gmail-Apple-converted-space"> </span>| <span class="gmail-Apple-converted-space"> </span>|</p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Menlo;font-size-adjust:none;font-kerning:auto;font-variant-alternates:normal;font-variant-ligatures:normal;font-variant-numeric:normal;font-variant-east-asian:normal;font-feature-settings:normal;color:rgb(93,108,121)"><span class="gmail-Apple-converted-space"> </span>0 +-------+------+-+-----------+-----------+</p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Menlo;font-size-adjust:none;font-kerning:auto;font-variant-alternates:normal;font-variant-ligatures:normal;font-variant-numeric:normal;font-variant-east-asian:normal;font-feature-settings:normal;color:rgb(93,108,121)"><span class="gmail-Apple-converted-space"> </span>|3<span class="gmail-Apple-converted-space"> </span>|4 <span class="gmail-Apple-converted-space"> </span>|5<span class="gmail-Apple-converted-space"> </span>|6<span class="gmail-Apple-converted-space"> </span>|</p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Menlo;font-size-adjust:none;font-kerning:auto;font-variant-alternates:normal;font-variant-ligatures:normal;font-variant-numeric:normal;font-variant-east-asian:normal;font-feature-settings:normal;color:rgb(93,108,121)"><span class="gmail-Apple-converted-space"> </span>| <span class="gmail-Apple-converted-space"> </span>|<span class="gmail-Apple-converted-space"> </span>| <span class="gmail-Apple-converted-space"> </span>| <span class="gmail-Apple-converted-space"> </span>|</p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Menlo;font-size-adjust:none;font-kerning:auto;font-variant-alternates:normal;font-variant-ligatures:normal;font-variant-numeric:normal;font-variant-east-asian:normal;font-feature-settings:normal;color:rgb(93,108,121)"><span class="gmail-Apple-converted-space"> </span>| <span class="gmail-Apple-converted-space"> </span>|<span class="gmail-Apple-converted-space"> </span>| <span class="gmail-Apple-converted-space"> </span>| <span class="gmail-Apple-converted-space"> </span>|</p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Menlo;font-size-adjust:none;font-kerning:auto;font-variant-alternates:normal;font-variant-ligatures:normal;font-variant-numeric:normal;font-variant-east-asian:normal;font-feature-settings:normal;color:rgb(93,108,121)"><span class="gmail-Apple-converted-space"> </span>| <span class="gmail-Apple-converted-space"> </span>|<span class="gmail-Apple-converted-space"> </span>| <span class="gmail-Apple-converted-space"> </span>| <span class="gmail-Apple-converted-space"> </span>|</p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Menlo;font-size-adjust:none;font-kerning:auto;font-variant-alternates:normal;font-variant-ligatures:normal;font-variant-numeric:normal;font-variant-east-asian:normal;font-feature-settings:normal;color:rgb(93,108,121)"><span class="gmail-Apple-converted-space"> </span>| <span class="gmail-Apple-converted-space"> </span>|<span class="gmail-Apple-converted-space"> </span>| <span class="gmail-Apple-converted-space"> </span>| <span class="gmail-Apple-converted-space"> </span>|</p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Menlo;font-size-adjust:none;font-kerning:auto;font-variant-alternates:normal;font-variant-ligatures:normal;font-variant-numeric:normal;font-variant-east-asian:normal;font-feature-settings:normal;color:rgb(93,108,121)"><span class="gmail-Apple-converted-space"> </span>+-------+--------+-----------+-----------+</p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Menlo;font-size-adjust:none;font-kerning:auto;font-variant-alternates:normal;font-variant-ligatures:normal;font-variant-numeric:normal;font-variant-east-asian:normal;font-feature-settings:normal;color:rgb(93,108,121)"><span class="gmail-Apple-converted-space"> </span>-180<span class="gmail-Apple-converted-space"> </span>-100<span class="gmail-Apple-converted-space"> </span>-20 <span class="gmail-Apple-converted-space"> </span>80<span class="gmail-Apple-converted-space"> </span>180</p>
<p style="margin:0px;font-stretch:normal;font-size:12px;line-height:normal;font-family:Menlo;font-size-adjust:none;font-kerning:auto;font-variant-alternates:normal;font-variant-ligatures:normal;font-variant-numeric:normal;font-variant-east-asian:normal;font-feature-settings:normal;color:rgb(93,108,121)">*/</p></div><div><br></div><div> </div><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Mon, 10 Mar 2025 at 20:47, Greg Troxel via PROJ <<a href="mailto:proj@lists.osgeo.org" target="_blank">proj@lists.osgeo.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-style:solid;border-left-color:rgb(204,204,204);padding-left:1ex">William Temperley via PROJ <<a href="mailto:proj@lists.osgeo.org" target="_blank">proj@lists.osgeo.org</a>> writes:<br>
<br>
> Couldn't the cutlines be reasonably easily generated in geographic<br>
> coordinates from either user-supplied or default projection<br>
> parameters, using one of the above methods?<br>
> An example for a derivable extent would be an orthographic projection where<br>
> the cutline is just 90 degrees in spherical distance from the centre point.<br>
<br>
A further question might be how the cutlines relate to the 'area of use'<br>
in the EPSG database, but I'm afraid the answer is that the area of use<br>
is a geodetic bounding box. It does seem that a projection database<br>
ideally would have this information.<br>
<br>
But, as you point out with Mercator, that leads to Mercator-80 and<br>
Mercator-85 entries, and that doesn't seem good.<br>
<br>
Not sure this ended up being helpful, but wanted to mention the possible<br>
relationship.<br>
<br>
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