How accurate are measurements? How far can coordinates be off?

A shape can have accurate dimensions and still be in the wrong place. The imagery you trace affects where your exported coordinates land.

This article explains the difference, shows how to check your own work, and gives two workflows for getting coordinates you can rely on.

The short answer

Scribble Maps calculates lengths and areas from the points you draw. How close those points are to the real world depends on two things:

The imagery you trace. Satellite and aerial imagery on any web map can be offset from the true ground position, and the amount changes from place to place.
How you trace. Image resolution and where you click each vertex limit how repeatable your trace is.

There is no single accuracy percentage or offset that applies to every map. To know how accurate your map is, you check it against something you trust.

Three different questions

"How accurate is it?" is really three separate questions. Each one is measured a different way.

Measurements (% error). Compare with known lengths or areas. Error depends on the source imagery, your tracing, the scale and the calculation method. One percentage cannot describe every map.
Precision (repeatability). Repeat the trace and see how much the results spread. Pixel resolution and vertex placement limit repeatability. More decimal places do not prove greater accuracy.
Position (offset in meters). Compare points you can identify at ground level against trusted control points, and report the displacement or the horizontal RMSE. Google imagery has no single offset that applies to every site.

Four targets showing accurate and precise, precise but not accurate, accurate but not precise, and neither

The most common surprise is "precise, not accurate". Every measurement agrees with the last one, so the map looks right, but the whole trace is offset from the real location.

Same dimensions, different location

Picture a 20 m by 10 m rectangle in its true location. Now picture the same rectangle traced on imagery that is shifted 3 m east and 4 m north. Both rectangles are 20 m by 10 m. The traced one is still 5 m from the true location.

A blue 20 m by 10 m rectangle at the true location and a gold dashed copy shifted 3 m east and 4 m north, a 5 m position error

A uniform local shift like this keeps the dimensions the same. Distortion in the imagery can change them.

Left: a uniform shift moves the shape without changing it. Right: distortion changes the shape's lengths and area

These figures are an illustration only. They are not measured Google offsets.

Why web map imagery can be off

Satellite and aerial imagery is processed and stitched together before it reaches any web map. A few common effects to watch for:

Tall objects lean. The camera sees buildings, towers and trees from an angle, so their tops can appear shifted away from where they stand. Trace where a building meets the ground, not its roof outline.
Imagery is a mosaic. One view can combine images from different dates and providers. The imagery credits at the bottom of the map often list several. Alignment can change where two images meet.
Resolution limits what you can see. You cannot place a vertex more precisely than the pixels you are clicking on.

Side view of a camera seeing a building at an angle so the roof appears beside it, and a top view showing the roof shifted from the ground-level footprint

How Scribble Maps calculates measurements

For step-by-step help, see the article on measuring area and distance.

Scribble Maps does not measure on the flat map you see on screen. Web maps stretch areas more the farther you get from the equator. Scribble Maps measures on a model of the Earth instead, so that stretching does not change your numbers.

The Earth model is a sphere with a radius of 6,378,137 m.
Line lengths are measured along the same path the line follows on the map, segment by segment.
Polygon areas are calculated on the sphere. Holes in a polygon are subtracted.
Circles use the radius from the center to the edge. The area is π × radius².

Survey and desktop GIS software often use an ellipsoid or a local projected coordinate system instead. Their results can differ slightly from a sphere-based measurement, especially over long distances.

Show enough decimal places

Measurements show 2 decimal places. With the default Auto setting, lengths are shown in kilometres (Metric) or miles (Imperial). A 37 m line reads "0.04 km". That rounding can hide several meters on a short line.

For detailed work, pick a smaller unit:

Select the gear icon in the bottom-left corner of the map to open Editor Settings.
Under Unit Types / Measurement, choose Metric or Imperial.
Set Length Unit to Metre or Feet, for example.
Set Area Unit to Square Metre or Square Foot if you need exact areas. In Auto, areas switch between square metres, hectares and square kilometres (or square feet, acres and square miles) based on size.

The Editor Settings panel with Metric selected and the Length Unit and Area Unit dropdowns set to Auto

Coordinates: the editor shows latitude and longitude to 5 decimal places, which is about 1 m. GeoJSON exports keep 9 decimal places. More decimal places describe the point you clicked more exactly. They do not make that point any closer to the real world.

Check against a length you already know

The easiest accuracy check is something with a known size. A football field is 100 yards (91.44 m) from goal line to goal line. Here is one trace of a field (the Rose Bowl in Pasadena, on Google satellite imagery), shown with two different Length Unit settings.

The same goal-line-to-goal-line trace showing Length 0.09 km with Length Unit Auto and Length 91.46 m with Length Unit Metre

With Auto, the tooltip shows 0.09 km. That is correct, but rounded to the nearest 10 m, so it is too rough to judge accuracy.
With Metre, it shows 91.46 m, within 0.02 m of the known length.

This is one trace of one field. It shows how to check your own work. It is not a guaranteed accuracy. Repeat the trace a few times to see how much your results spread (precision), and check a known point to see whether the imagery is shifted (position).

A practical percentage example (not a product tolerance)

Assume each end of a line is within 0.25 m of its true position. The worst-case tracing error is:

About 0.5% over 100 m
About 2.5% over 20 m

The formula is 2 × 0.25 ÷ length × 100.

A chart of worst-case length error against line length, showing 2.5% at 20 m and 0.5% at 100 m

The same tracing error is a bigger percentage on a short line. Imagery and calculation errors must be checked separately. Area error depends on the shape and on where the boundary is placed.

Two workflows for reliable alignment

Workflow 1: export, check coordinate system, compare with control, then move by a verified offset or retrace. Workflow 2: get georeferenced imagery, load the GeoTIFF, check alignment, trace and export

1) Correct in your GIS software

Already traced on a base map?

Export your features from Scribble Maps. Open the Menu, then Import & Export, then Export As, and pick a format such as GeoJSON, Shapefile (.SHP), GeoPackage (.GPKG), KML or DXF. Exporting requires an upgraded plan.
Check the coordinate system and datum first. GeoJSON and most Export As formats use WGS84 latitude and longitude (EPSG:4326). DXF and DGN files are projected into the UTM zone at the center of your data. When you export DXF, choosing Illustrator uses Web Mercator instead.
In your GIS software, compare your features with trusted imagery or control points.
If the shift is the same everywhere, use your GIS software's move tools to apply the offset you verified.
If the error varies from place to place, the features need more correction or retracing.

2) Trace on a GeoTIFF

Need aligned coordinates from the start?

Use your organization's imagery, or order orthorectified, georeferenced imagery with documented positional accuracy.
In Scribble Maps, select the Image Tool in the toolbar. The Image Overlay panel opens. You need to be logged in to upload imagery.
Choose your GeoTIFF with Choose Files, or drop it on Drag image here. When the file is georeferenced, Scribble Maps reads its location from the file and reprojects it to fit the map. The North, South, East and West boxes fill in for you.
Use the Opacity slider to see the base map through your image, and check its alignment against known points.
Select Place On Map, trace on your imagery, then export to your GIS software.

The Image Overlay panel with Upload file, Choose Files, Drag image here and Place On Map

Key takeaway

Accurate dimensions do not automatically mean accurate coordinates. Use trusted imagery, check alignment, and validate against ground control.

This article does not quote a single verified Scribble Maps accuracy percentage or a Google positional tolerance, because no universal figure applies. A GeoTIFF carries spatial referencing, not an accuracy guarantee. For each project, validate the imagery, ground control, datum transformation and measurement method.

Last updated October 13, 2026