diff --git a/src/annotation/annotation.js b/src/annotation/annotation.js
index ef7e71c316..b162d5860b 100644
--- a/src/annotation/annotation.js
+++ b/src/annotation/annotation.js
@@ -1358,10 +1358,20 @@ function continuousVerticesProcessAction(m_this, evt, name) {
  *   wide or half as wide as it is tall.  Sizes (e.g., {width: 400, height:
  *   500}) snap to that size.
  * @returns {Function} A function that can be passed to the mapIterator
- *   selectionConstraint or to an annotation constraint function.
+ *   selectionConstraint or to an annotation constraint function.  If any sizes
+ *   are given, the returned function is stateful: it remembers where the
+ *   fixed-size box was left so that the box only moves once the mouse reaches
+ *   its edge.  That state is reset whenever the origin changes, so build a
+ *   separate function per shape being drawn rather than sharing one.
  */
 function constrainAspectRatio(ratio) {
   const ratios = Array.isArray(ratio) ? ratio : [ratio];
+  /* A fixed-size box lags the mouse: it holds still while the mouse is inside it and is only
+   * pushed along once the mouse reaches an edge.  That is path-dependent, so the center of the
+   * last box is remembered here.  It is reset whenever a new action begins, which is detected by
+   * a change of origin.  Each annotation builds its own constraint function, so this state is
+   * scoped to a single shape being drawn. */
+  let lastOrigin, lastCenter;
 
   /**
    * Constrain a mouse action or annotation action to a list of aspect ratios.
@@ -1459,6 +1469,13 @@ function constrainAspectRatio(ratio) {
     } else {
       /* Not in edit vertex or edge mode */
       const area = Math.abs((pos.x - origin.x) * (pos.y - origin.y));
+      let anchor = origin;
+      if (!lastOrigin || lastOrigin.x !== origin.x || lastOrigin.y !== origin.y) {
+        lastOrigin = {x: origin.x, y: origin.y};
+        lastCenter = {x: origin.x, y: origin.y};
+      }
+      /* Only the winning ratio's center may be committed, so hold it aside until the loop ends. */
+      let bestCenter;
       ratios.forEach((ratio) => {
         let width, height;
         if (ratio.width) {
@@ -1468,18 +1485,48 @@ function constrainAspectRatio(ratio) {
           width = (area * ratio) ** 0.5;
           height = width / ratio;
         }
-        const adjusted = {
+        const originAnchored = {
           x: origin.x + Math.sign(pos.x - origin.x) * width,
           y: origin.y + Math.sign(pos.y - origin.y) * height
         };
-        const score = (adjusted.x - pos.x) ** 2 + (adjusted.y - pos.y) ** 2;
+        const score = (originAnchored.x - pos.x) ** 2 + (originAnchored.y - pos.y) ** 2;
         if (best === undefined || score < best) {
           best = score;
-          newpos = adjusted;
+          if (ratio.width) {
+            /* Fixed-size shapes have no remaining degree of freedom to resize, so dragging moves
+             * the box rather than resizing it.  It starts centered on the origin and only gives
+             * way once the mouse reaches an edge: clamping the previous center to within half a
+             * box of the mouse leaves it untouched while the mouse is inside, and otherwise
+             * pushes it just far enough to keep the mouse on the edge.  Each axis is clamped
+             * independently, so moving the mouse off the right edge slides the box sideways
+             * without disturbing it vertically. */
+            const center = {
+              x: Math.min(Math.max(lastCenter.x, pos.x - width / 2), pos.x + width / 2),
+              y: Math.min(Math.max(lastCenter.y, pos.y - height / 2), pos.y + height / 2)
+            };
+            bestCenter = center;
+            /* `anchor` is the upper-left corner and `newpos` the lower-right; map gcs has y
+             * increasing upwards, so the upper corner is the larger y. */
+            anchor = {
+              x: center.x - width / 2,
+              y: center.y + height / 2
+            };
+            newpos = {
+              x: center.x + width / 2,
+              y: center.y - height / 2
+            };
+          } else {
+            bestCenter = undefined;
+            anchor = origin;
+            newpos = originAnchored;
+          }
         }
       });
-      corners[0].y = corners[1].y = origin.y;
-      corners[0].x = corners[3].x = origin.x;
+      if (bestCenter) {
+        lastCenter = bestCenter;
+      }
+      corners[0].y = corners[1].y = anchor.y;
+      corners[0].x = corners[3].x = anchor.x;
       corners[1].x = corners[2].x = newpos.x;
       corners[2].y = corners[3].y = newpos.y;
     }
diff --git a/src/annotation/rectangleAnnotation.js b/src/annotation/rectangleAnnotation.js
index ee3e9ee17d..264a7d9b2c 100644
--- a/src/annotation/rectangleAnnotation.js
+++ b/src/annotation/rectangleAnnotation.js
@@ -22,10 +22,15 @@ const annotationActionOwner = require('./annotation').annotationActionOwner;
  *    finished rectangle.  This uses styles for {@link geo.polygonFeature}.
  * @property {geo.polygonFeature.styleSpec} [editStyle] The style to apply to a
  *    rectangle in edit mode.
- * @property {number|number[]|Function} [constraint] If specified, an aspect
- *    ratio or list of aspect ratios to constraint the rectangle to.  If a
- *    function, a selection constraint function to call to adjust the
- *    rectangle.
+ * @property {number|geo.geoSize|Array.<number|geo.geoSize>|Function} [constraint]
+ *    If specified, an aspect ratio, a fixed size, or a list of allowed aspect ratios and sizes to
+ *    constrain the rectangle to. A number (or a list of numbers) is an aspect ratio - the
+ *    rectangle is resized as it is drawn, with the initial click point fixed as one corner. A size
+ *    (e.g., `{width: 20, height: 10}`) instead fixes the rectangle's dimensions - since there is
+ *    no remaining degree of freedom to resize, dragging translates the whole rectangle rather
+ *    than resizing it: it starts centered on the initial click and then stays put while the
+ *    mouse moves inside it, only being pushed along once the mouse reaches one of its edges. If
+ *    a function, a selection constraint function to call to adjust the rectangle.
  */
 
 /**
@@ -55,7 +60,9 @@ var rectangleAnnotation = function (args, annotationName) {
 
   var m_this = this,
       s_actions = this.actions,
-      s_processEditAction = this.processEditAction;
+      s_processEditAction = this.processEditAction,
+      // The original click location that started the current draw.
+      m_origin = null;
 
   /**
    * Return actions needed for the specified state of this annotation.
@@ -234,7 +241,8 @@ var rectangleAnnotation = function (args, annotationName) {
     corners[1] = map.displayToGcs(c1, null);
     corners[3] = map.displayToGcs(c3, null);
     if (this._selectionConstraint) {
-      this._selectionConstraint(evt.mapgcs, corners[0], corners);
+      // Use the origin recorded at the start of the draw.
+      this._selectionConstraint(evt.mapgcs, m_origin || corners[0], corners);
     }
   };
 
@@ -291,10 +299,13 @@ var rectangleAnnotation = function (args, annotationName) {
       return 'done';
     }
     if (evt.buttonsDown.left) {
+      m_origin = Object.assign({}, evt.mapgcs);
       corners.push(Object.assign({}, evt.mapgcs));
       corners.push(Object.assign({}, evt.mapgcs));
       corners.push(Object.assign({}, evt.mapgcs));
       corners.push(Object.assign({}, evt.mapgcs));
+      // Apply the constraint immediately so fixed-size shapes show right away.
+      m_this._setCornersFromMouse(corners, evt);
       return true;
     }
     return undefined;
diff --git a/src/mapInteractor.js b/src/mapInteractor.js
index 5ce5ddce1c..9d0f07918d 100644
--- a/src/mapInteractor.js
+++ b/src/mapInteractor.js
@@ -3,6 +3,10 @@ var object = require('./object');
 var util = require('./util');
 var Mousetrap = require('mousetrap');
 
+/* A rectangle's corners array must have at least this many points before its
+ * first corner can be trusted as a valid anchor point. */
+var MIN_ANCHOR_CORNERS = 3;
+
 /**
  * Map Interactor specification.
  *
@@ -989,9 +993,15 @@ var mapInteractor = function (args) {
         display = {}, gcs = {};
 
     let mousexy = mouse.map;
+    let anchorxy = origin.map;
     if (m_state.actionRecord && util.isFunction(m_state.actionRecord.selectionConstraint)) {
       const constraint = m_state.actionRecord.selectionConstraint(mouse.mapgcs, origin.mapgcs);
-      mousexy = constraint ? map.gcsToDisplay(constraint.pos, null) : mousexy;
+      if (constraint) {
+        mousexy = map.gcsToDisplay(constraint.pos, null);
+        if (constraint.corners && constraint.corners.length >= MIN_ANCHOR_CORNERS) {
+          anchorxy = map.gcsToDisplay(constraint.corners[0], null);
+        }
+      }
     } else if (mouse.modifiers.shift) {
       const width =  Math.abs((mousexy.x - origin.map.x) * (mousexy.y - origin.map.y)) ** 0.5;
       mousexy = {
@@ -1001,13 +1011,13 @@ var mapInteractor = function (args) {
     }
     // Get the display coordinates
     display.upperLeft = {
-      x: Math.min(origin.map.x, mousexy.x),
-      y: Math.min(origin.map.y, mousexy.y)
+      x: Math.min(anchorxy.x, mousexy.x),
+      y: Math.min(anchorxy.y, mousexy.y)
     };
 
     display.lowerRight = {
-      x: Math.max(origin.map.x, mousexy.x),
-      y: Math.max(origin.map.y, mousexy.y)
+      x: Math.max(anchorxy.x, mousexy.x),
+      y: Math.max(anchorxy.y, mousexy.y)
     };
 
     display.upperRight = {
diff --git a/tests/cases/annotation.js b/tests/cases/annotation.js
index 6dae308e4d..4327bdcc50 100644
--- a/tests/cases/annotation.js
+++ b/tests/cases/annotation.js
@@ -928,6 +928,32 @@ describe('geo.annotation', function () {
     });
   });
 
+  describe('geo.annotation.rectangleAnnotation with mixed aspect ratio and fixed size', function () {
+    it('does not let a fixed-size selection drift the origin used for later aspect-ratio checks', function () {
+      var map = createMap();
+      var layer = map.createLayer('annotation', {annotations: ['rectangle']});
+      var ann = geo.annotation.rectangleAnnotation({
+        layer: layer,
+        constraint: [2, {width: 10, height: 10}]
+      });
+      ann.state(geo.annotation.state.create);
+      // First click establishes the true origin at (0, 0).
+      ann.mouseClick({
+        buttonsDown: {left: true},
+        time: Date.now(),
+        map: {x: 0, y: 0},
+        mapgcs: {x: 0, y: 0}
+      });
+      ann.mouseMove({mapgcs: {x: 11, y: 9}});
+      ann.mouseMove({mapgcs: {x: 40, y: 20}});
+      var corners = ann.options('corners');
+      expect([corners[0].x, corners[0].y]).toEqual([0, 0]);
+      expect([corners[1].x, corners[1].y]).toEqual([40, 0]);
+      expect([corners[2].x, corners[2].y]).toEqual([40, 20]);
+      expect([corners[3].x, corners[3].y]).toEqual([0, 20]);
+    });
+  });
+
   describe('geo.annotation.polygonAnnotation', function () {
     var vertices = [{x: 30, y: 0}, {x: 50, y: 0}, {x: 40, y: 20}, {x: 30, y: 10}];
     var vertices2 = [{x: 30, y: 10}, {x: 50, y: 10}, {x: 40, y: 30}];
@@ -1682,20 +1708,29 @@ describe('geo.annotation', function () {
       const func = geo.annotation.constrainAspectRatio({width: 20, height: 10});
 
       let result;
+      // A fixed-size shape anchors at the current mouse position (not origin) and always extends
+      // in the same direction, so it translates with the cursor instead of flipping across
+      // quadrants.
       result = func(
         {x: 40, y: 5},
         {x: 0, y: 0});
-      expect(result.pos).toEqual({x: 20, y: 10});
+      expect(result.pos).toEqual({x: 60, y: 15});
+      expect(result.corners).toEqual([{x: 40, y: 5}, {x: 60, y: 5}, {x: 60, y: 15}, {x: 40, y: 15}]);
       result = func(
         {x: 40, y: 5},
         {x: 0, y: 0},
         [{x: 0, y: 0}, {x: 10, y: 0}, {x: 10, y: 10}, {x: 0, y: 10}]);
-      expect(result.pos).toEqual({x: 20, y: 10});
+      expect(result.pos).toEqual({x: 60, y: 15});
       result = func(
         {x: 5, y: 40},
         {x: 0, y: 0},
         [{x: 0, y: 0}, {x: 10, y: 0}, {x: 10, y: 10}, {x: 0, y: 10}]);
-      expect(result.pos).toEqual({x: 20, y: 10});
+      expect(result.pos).toEqual({x: 25, y: 50});
+      // Moving the mouse to the opposite side of origin must not flip the extension direction.
+      result = func(
+        {x: -40, y: -5},
+        {x: 0, y: 0});
+      expect(result.pos).toEqual({x: -20, y: 5});
       result = func(
         {x: 0, y: 0},
         {x: 0, y: 0},
@@ -1709,6 +1744,18 @@ describe('geo.annotation', function () {
         'vertex', [0, -Math.PI / 2, Math.PI, Math.PI / 2], 0);
       expect(result.corners).toEqual([{x: 10, y: 20}, {x: 30, y: 20}, {x: 30, y: 10}, {x: 10, y: 10}]);
     });
+    it('multiple fixed sizes picks the closest match', function () {
+      // With multiple fixed sizes in the list, the one closest to the actual drag distance
+      // must be chosen, not simply the first fixed-size entry.
+      const func = geo.annotation.constrainAspectRatio([
+        {width: 100, height: 100}, {width: 10, height: 10}]);
+
+      let result;
+      result = func({x: 11, y: 11}, {x: 0, y: 0});
+      expect(result.pos).toEqual({x: 21, y: 21});
+      result = func({x: 95, y: 95}, {x: 0, y: 0});
+      expect(result.pos).toEqual({x: 195, y: 195});
+    });
   });
 
   describe('annotation registry', function () {
diff --git a/tests/cases/mapInteractor.js b/tests/cases/mapInteractor.js
index 316dc4c215..cc26d2bb91 100644
--- a/tests/cases/mapInteractor.js
+++ b/tests/cases/mapInteractor.js
@@ -652,6 +652,52 @@ describe('mapInteractor', function () {
     expect(clickTriggered).toBe(1);
   });
 
+  it('Test _getSelection uses the selectionConstraint anchor corner', function () {
+    var map = mockedMap('#mapNode1'),
+        selection;
+
+    // A selectionConstraint that mimics a fixed-size annotation: the anchor corner always tracks
+    // the current mouse position, and the opposite corner is a constant offset away.
+    var interactor = geo.mapInteractor({
+      map: map,
+      actions: [{
+        action: geo.geo_action.select,
+        input: 'left',
+        selectionRectangle: geo.event.select,
+        selectionConstraint: function (pos) {
+          return {
+            pos: {x: pos.x + 10, y: pos.y + 5},
+            corners: [
+              {x: pos.x, y: pos.y},
+              {x: pos.x + 10, y: pos.y},
+              {x: pos.x + 10, y: pos.y + 5},
+              {x: pos.x, y: pos.y + 5}
+            ]
+          };
+        }
+      }],
+      throttle: false
+    });
+    map.geoOn(geo.event.select, function (evt) {
+      selection = evt;
+    });
+
+    interactor.simulateEvent(
+      'mousedown', {map: {x: 20, y: 20}, button: 'left'}
+    );
+    interactor.simulateEvent(
+      'mousemove.geojs', {map: {x: 50, y: 50}, button: 'left'}
+    );
+    interactor.simulateEvent(
+      'mouseup.geojs', {map: {x: 50, y: 50}, button: 'left'}
+    );
+
+    // The selection box should be anchored at the current mouse position, not at the original
+    // mousedown point.
+    expect(selection.display.upperLeft).toEqual({x: 50, y: 50});
+    expect(selection.display.lowerRight).toEqual({x: 60, y: 55});
+  });
+
   describe('pause state', function () {
     it('defaults', function () {
       expect(geo.mapInteractor().pause()).toBe(false);
diff --git a/tutorials/annotation/index.pug b/tutorials/annotation/index.pug
index 7b544a5e11..42fd4d0746 100644
--- a/tutorials/annotation/index.pug
+++ b/tutorials/annotation/index.pug
@@ -3,7 +3,7 @@ extends ../common/index.pug
 block mainTutorial
   :markdown-it
     # Tutorial - Annotations
-    Draw different annotations, optionally setting the aspect ratio for rectangles and ellipses.
+    Draw different annotations, optionally setting the aspect ratio or a fixed size for rectangles and ellipses.
 
     Define some HTML with the divs for the map and controls.
 
@@ -26,6 +26,7 @@ block mainTutorial
         <option id="point">Point</option>
         </select>
         Aspect Ratio <input id="aspect" size="3"></input>
+        Fixed Size (w,h) <input id="fixedsize" size="7"></input>
         <button id="draw">Draw</button>
       </div>
     </body>
@@ -106,6 +107,47 @@ block mainTutorial
       }
     });
 
+  :markdown-it
+    Rectangles and ellipses can also be constrained to a *fixed size* rather than an aspect ratio, using the "Fixed Size" field. `square` and `circle` always use a fixed aspect ratio of 1 and cannot be given a fixed size, so the field only applies to `rectangle` and `ellipse`.
+
+    Unlike an aspect-ratio constraint, a fixed-size shape has no remaining degree of freedom to resize as you drag - it is always exactly the size you specify. Because of this, dragging *moves* the shape instead of resizing it: the current mouse position becomes the rectangle's corner, and the rest of the rectangle extends from there in a fixed direction, so it translates smoothly with the cursor.
+
+  +codeblock('javascript', 5, 3).
+    $('#draw').on('click', () => {
+      if ($('#draw').hasClass('drawing')) {
+        // turn off drawing an annotation
+        annotLayer.mode(null);
+        $('#draw').removeClass('drawing');
+      } else {
+        // mode is the type of anntoation to draw
+        var mode = $('#mode option:selected').attr('id');
+        var opts = {};
+        if (mode === 'rectangle' || mode === 'ellipse') {
+          // if specified, a fixed width and height (comma-separated) take
+          // precedence over the aspect ratio
+          var fixedsize = $('#fixedsize').val().trim();
+          if (fixedsize) {
+            var size = fixedsize.split(',').map((v) => parseFloat(v));
+            opts.constraint = {width: size[0], height: size.length > 1 ? size[1] : size[0]};
+          } else {
+            // otherwise, if specified, set the aspect ratio constraint to a
+            // list of values
+            var aspect = $('#aspect').val().trim();
+            if (aspect) {
+              opts.constraint = aspect.split(',').map((v) => parseFloat(v));
+            }
+          }
+        }
+        // switch to drawing mode
+        annotLayer.mode(mode, undefined, opts);
+        $('#draw').addClass('drawing');
+        // when finished, turn off the button style so it is obvious
+        annotLayer.geoOnce(geo.event.annotation.mode, () => {
+          $('#draw').removeClass('drawing');
+        });
+      }
+    });
+
   +codeblock_test('map has an annotation layer', [
     'map.layers().length === 2',
     'map.layers()[1] instanceof geo.annotationLayer',