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Unable to Get Current Map Center & Zoom Level

SteveEllis1

I am using the ArcGIS 100.6 iOS SDK and am trying to determine the current center of the map view in lat/long coordinates and the view area. I also want to invoke a listener to detect when a user pans of zooms the map to get the new center and view area. I am need this information to create and update queries to Firebase/GeoFire so that I can place the appropriate markers on the map based on the viewable area. As the user zooms out more markers should become visible.

Google maps has a simple way to do this: center = map.getCenter.

I cannot find a simple answer to this in the SDK documentation.

I have tried setting currentCenter = mapView.center, but the result is in screen coordinates and there is no concise documentation as to how to convert this point to lat/long.

Thanks,

Steve

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Accepted answers

NFurness

There are a few approaches you can take.

1. Read the current viewpoint as a Center and Scale and use the targetGeometry (it will be a point):

<SPAN class="keyword token">let</SPAN> mapCenter <SPAN class="operator token">=</SPAN> mapView<SPAN class="punctuation token">.</SPAN><SPAN class="token function">currentViewpoint</SPAN><SPAN class="punctuation token">(</SPAN><SPAN class="keyword token">with</SPAN><SPAN class="punctuation token">:</SPAN> <SPAN class="punctuation token">.</SPAN>centerAndScale<SPAN class="punctuation token">)</SPAN><SPAN class="operator token">?</SPAN><SPAN class="punctuation token">.</SPAN>targetGeometry <SPAN class="keyword token">as</SPAN><SPAN class="operator token">?</SPAN> AGSPoint<SPAN class="line-numbers-rows"><SPAN>‍</SPAN></SPAN>

2. Get the UI center (as you were) and convert it to map coordinates:

<SPAN class="keyword token">let</SPAN> mapCenter <SPAN class="operator token">=</SPAN> mapView<SPAN class="punctuation token">.</SPAN><SPAN class="token function">screen</SPAN><SPAN class="punctuation token">(</SPAN>toLocation<SPAN class="punctuation token">:</SPAN> mapView<SPAN class="punctuation token">.</SPAN>center<SPAN class="punctuation token">)</SPAN>
<SPAN class="line-numbers-rows"><SPAN>‍</SPAN></SPAN>

3. Use the visibleArea property to derive the center:

<SPAN class="keyword token">let</SPAN> mapCenter <SPAN class="operator token">=</SPAN> mapView<SPAN class="punctuation token">.</SPAN>visibleArea<SPAN class="operator token">?</SPAN><SPAN class="punctuation token">.</SPAN>extent<SPAN class="punctuation token">.</SPAN>center<SPAN class="line-numbers-rows"><SPAN>‍</SPAN></SPAN>

In all those cases, the coordinate will be in the map's spatial reference. You'll want to convert them to lat/lon like this:

<SPAN class="keyword token">if</SPAN> <SPAN class="keyword token">let</SPAN> latLon <SPAN class="operator token">=</SPAN> AGSGeometryEngine<SPAN class="punctuation token">.</SPAN><SPAN class="token function">projectGeometry</SPAN><SPAN class="punctuation token">(</SPAN>mapCenter<SPAN class="punctuation token">,</SPAN> to<SPAN class="punctuation token">:</SPAN> <SPAN class="punctuation token">.</SPAN><SPAN class="token function">wgs84</SPAN><SPAN class="punctuation token">(</SPAN><SPAN class="punctuation token">)</SPAN><SPAN class="punctuation token">)</SPAN> <SPAN class="keyword token">as</SPAN><SPAN class="operator token">?</SPAN> AGSPoint <SPAN class="punctuation token">{</SPAN>
    <SPAN class="keyword token">let</SPAN> lat <SPAN class="operator token">=</SPAN> latLon<SPAN class="punctuation token">.</SPAN>y
    <SPAN class="keyword token">let</SPAN> lon <SPAN class="operator token">=</SPAN> latLon<SPAN class="punctuation token">.</SPAN>x
<SPAN class="punctuation token">}</SPAN><SPAN class="line-numbers-rows"><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN></SPAN>

If you find yourself needing latitude and longitude a lot, you can create an extension on AGSPoint:

extension AGSPoint <SPAN class="punctuation token">{</SPAN>
    <SPAN class="keyword token">var</SPAN> latitude<SPAN class="punctuation token">:</SPAN> Double <SPAN class="punctuation token">{</SPAN>
        <SPAN class="keyword token">return</SPAN> <SPAN class="punctuation token">(</SPAN>AGSGeometryEngine<SPAN class="punctuation token">.</SPAN><SPAN class="token function">projectGeometry</SPAN><SPAN class="punctuation token">(</SPAN>self<SPAN class="punctuation token">,</SPAN> to<SPAN class="punctuation token">:</SPAN> <SPAN class="punctuation token">.</SPAN><SPAN class="token function">wgs84</SPAN><SPAN class="punctuation token">(</SPAN><SPAN class="punctuation token">)</SPAN><SPAN class="punctuation token">)</SPAN> <SPAN class="keyword token">as</SPAN><SPAN class="operator token">?</SPAN> AGSPoint<SPAN class="punctuation token">)</SPAN><SPAN class="operator token">?</SPAN><SPAN class="punctuation token">.</SPAN>y <SPAN class="operator token">?</SPAN><SPAN class="operator token">?</SPAN> <SPAN class="number token">0</SPAN>
    <SPAN class="punctuation token">}</SPAN>

    <SPAN class="keyword token">var</SPAN> longitude<SPAN class="punctuation token">:</SPAN> Double <SPAN class="punctuation token">{</SPAN>
        <SPAN class="keyword token">return</SPAN> <SPAN class="punctuation token">(</SPAN>AGSGeometryEngine<SPAN class="punctuation token">.</SPAN><SPAN class="token function">projectGeometry</SPAN><SPAN class="punctuation token">(</SPAN>self<SPAN class="punctuation token">,</SPAN> to<SPAN class="punctuation token">:</SPAN> <SPAN class="punctuation token">.</SPAN><SPAN class="token function">wgs84</SPAN><SPAN class="punctuation token">(</SPAN><SPAN class="punctuation token">)</SPAN><SPAN class="punctuation token">)</SPAN> <SPAN class="keyword token">as</SPAN><SPAN class="operator token">?</SPAN> AGSPoint<SPAN class="punctuation token">)</SPAN><SPAN class="operator token">?</SPAN><SPAN class="punctuation token">.</SPAN>x <SPAN class="operator token">?</SPAN><SPAN class="operator token">?</SPAN> <SPAN class="number token">0</SPAN>
    <SPAN class="punctuation token">}</SPAN>
<SPAN class="punctuation token">}</SPAN>
<SPAN class="line-numbers-rows"><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN></SPAN>

Note that if you're just using the Lat/Lon for display, you could make use of AGSCoordinateFormatter.LatitudeLongitudeStringFromPoint:format:decimalPlaces:()

As for detecting when there's a new center, there have been a couple of questions about this recently which should help you:

Hope this helps!

All comments

NFurness

Hi Steve. Maybe

It depends on the Basemap. Runtime determines the spatial reference to use by looking at the Basemap first (or if there isn't a basemap it has some fallback logic). Most likely your Basemap will be in Web Mercator, and so all coordinates for retrieved features ‌will be in Web Mercator too. This is to avoid having the Runtime re-project from source data every time it needs to draw things. That can rapidly affect performance.

If you use a WGS84 Basemap, you will see coordinates in Lat Lon.

Generally, my previous suggestions will be more flexible and won't lock you to one basemap spatial reference or another.

Also note that if you provide points created with AGSPointMakeWGS84() (or the standard AGSPoint constructor, specifying a Spatial Reference of .wgs84()) then you can provide those to set the viewpoint, etc. and Runtime will convert them appropriately. Really, your issue will typically only arise when reading coordinates and displaying them to the user (also when debugging and sanity checking against known latitudes and longitudes).

SteveEllis1

Thanks, Nick.

Just as a follow-up since my app only uses Lat/Long can I set the target geometry for my maps so that all map operations are in Lat/Long and not have to change spatial references?

NFurness

There are a few approaches you can take.

1. Read the current viewpoint as a Center and Scale and use the targetGeometry (it will be a point):

<SPAN class="keyword token">let</SPAN> mapCenter <SPAN class="operator token">=</SPAN> mapView<SPAN class="punctuation token">.</SPAN><SPAN class="token function">currentViewpoint</SPAN><SPAN class="punctuation token">(</SPAN><SPAN class="keyword token">with</SPAN><SPAN class="punctuation token">:</SPAN> <SPAN class="punctuation token">.</SPAN>centerAndScale<SPAN class="punctuation token">)</SPAN><SPAN class="operator token">?</SPAN><SPAN class="punctuation token">.</SPAN>targetGeometry <SPAN class="keyword token">as</SPAN><SPAN class="operator token">?</SPAN> AGSPoint<SPAN class="line-numbers-rows"><SPAN>‍</SPAN></SPAN>

2. Get the UI center (as you were) and convert it to map coordinates:

<SPAN class="keyword token">let</SPAN> mapCenter <SPAN class="operator token">=</SPAN> mapView<SPAN class="punctuation token">.</SPAN><SPAN class="token function">screen</SPAN><SPAN class="punctuation token">(</SPAN>toLocation<SPAN class="punctuation token">:</SPAN> mapView<SPAN class="punctuation token">.</SPAN>center<SPAN class="punctuation token">)</SPAN>
<SPAN class="line-numbers-rows"><SPAN>‍</SPAN></SPAN>

3. Use the visibleArea property to derive the center:

<SPAN class="keyword token">let</SPAN> mapCenter <SPAN class="operator token">=</SPAN> mapView<SPAN class="punctuation token">.</SPAN>visibleArea<SPAN class="operator token">?</SPAN><SPAN class="punctuation token">.</SPAN>extent<SPAN class="punctuation token">.</SPAN>center<SPAN class="line-numbers-rows"><SPAN>‍</SPAN></SPAN>

In all those cases, the coordinate will be in the map's spatial reference. You'll want to convert them to lat/lon like this:

<SPAN class="keyword token">if</SPAN> <SPAN class="keyword token">let</SPAN> latLon <SPAN class="operator token">=</SPAN> AGSGeometryEngine<SPAN class="punctuation token">.</SPAN><SPAN class="token function">projectGeometry</SPAN><SPAN class="punctuation token">(</SPAN>mapCenter<SPAN class="punctuation token">,</SPAN> to<SPAN class="punctuation token">:</SPAN> <SPAN class="punctuation token">.</SPAN><SPAN class="token function">wgs84</SPAN><SPAN class="punctuation token">(</SPAN><SPAN class="punctuation token">)</SPAN><SPAN class="punctuation token">)</SPAN> <SPAN class="keyword token">as</SPAN><SPAN class="operator token">?</SPAN> AGSPoint <SPAN class="punctuation token">{</SPAN>
    <SPAN class="keyword token">let</SPAN> lat <SPAN class="operator token">=</SPAN> latLon<SPAN class="punctuation token">.</SPAN>y
    <SPAN class="keyword token">let</SPAN> lon <SPAN class="operator token">=</SPAN> latLon<SPAN class="punctuation token">.</SPAN>x
<SPAN class="punctuation token">}</SPAN><SPAN class="line-numbers-rows"><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN></SPAN>

If you find yourself needing latitude and longitude a lot, you can create an extension on AGSPoint:

extension AGSPoint <SPAN class="punctuation token">{</SPAN>
    <SPAN class="keyword token">var</SPAN> latitude<SPAN class="punctuation token">:</SPAN> Double <SPAN class="punctuation token">{</SPAN>
        <SPAN class="keyword token">return</SPAN> <SPAN class="punctuation token">(</SPAN>AGSGeometryEngine<SPAN class="punctuation token">.</SPAN><SPAN class="token function">projectGeometry</SPAN><SPAN class="punctuation token">(</SPAN>self<SPAN class="punctuation token">,</SPAN> to<SPAN class="punctuation token">:</SPAN> <SPAN class="punctuation token">.</SPAN><SPAN class="token function">wgs84</SPAN><SPAN class="punctuation token">(</SPAN><SPAN class="punctuation token">)</SPAN><SPAN class="punctuation token">)</SPAN> <SPAN class="keyword token">as</SPAN><SPAN class="operator token">?</SPAN> AGSPoint<SPAN class="punctuation token">)</SPAN><SPAN class="operator token">?</SPAN><SPAN class="punctuation token">.</SPAN>y <SPAN class="operator token">?</SPAN><SPAN class="operator token">?</SPAN> <SPAN class="number token">0</SPAN>
    <SPAN class="punctuation token">}</SPAN>

    <SPAN class="keyword token">var</SPAN> longitude<SPAN class="punctuation token">:</SPAN> Double <SPAN class="punctuation token">{</SPAN>
        <SPAN class="keyword token">return</SPAN> <SPAN class="punctuation token">(</SPAN>AGSGeometryEngine<SPAN class="punctuation token">.</SPAN><SPAN class="token function">projectGeometry</SPAN><SPAN class="punctuation token">(</SPAN>self<SPAN class="punctuation token">,</SPAN> to<SPAN class="punctuation token">:</SPAN> <SPAN class="punctuation token">.</SPAN><SPAN class="token function">wgs84</SPAN><SPAN class="punctuation token">(</SPAN><SPAN class="punctuation token">)</SPAN><SPAN class="punctuation token">)</SPAN> <SPAN class="keyword token">as</SPAN><SPAN class="operator token">?</SPAN> AGSPoint<SPAN class="punctuation token">)</SPAN><SPAN class="operator token">?</SPAN><SPAN class="punctuation token">.</SPAN>x <SPAN class="operator token">?</SPAN><SPAN class="operator token">?</SPAN> <SPAN class="number token">0</SPAN>
    <SPAN class="punctuation token">}</SPAN>
<SPAN class="punctuation token">}</SPAN>
<SPAN class="line-numbers-rows"><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN><SPAN>‍</SPAN></SPAN>

Note that if you're just using the Lat/Lon for display, you could make use of AGSCoordinateFormatter.LatitudeLongitudeStringFromPoint:format:decimalPlaces:()

As for detecting when there's a new center, there have been a couple of questions about this recently which should help you:

Hope this helps!

MichaelDavis3

I use AGSViewpoint to get the current extent of the map:

AGSViewpoint *viewpoint = [AGSViewpoint viewpointWithTargetExtent:self.mapView.visibleArea.extent rotation:self.mapView.rotation];

Once you have that I think you can get the center point using viewpoint.targetGeometry.extent.center.x and viewpoint.targetGeometry.extent.center.y

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